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97-433: The 445-square-kilometre (172 sq mi) municipality is the 225th largest by area out of Norway's 357 municipalities and the 49th most populous, with a population of 25,056. The municipality's population density is 56.3 inhabitants per square kilometre (146/sq mi) and the population has increased by 2.5% over the previous 10-year period. The municipality is located on many islands in southern Troms county. Most of

194-401: A mural crown is typically shown above the shield . The arms were designed by Jardar Lunde in cooperation with Hallvard Træteberg. Harstad Municipality is responsible for primary education (through 10th grade), outpatient health services , senior citizen services, welfare and other social services , zoning , economic development , and municipal roads and utilities. The municipality

291-573: A fast pace. Cities with high population densities are, by some, considered to be overpopulated, though this will depend on factors like quality of housing and infrastructure and access to resources. Very densely populated cities are mostly in Asia (particularly Southeast Asia ); Africa's Lagos , Kinshasa , and Cairo ; South America's Bogotá , Lima , and São Paulo ; and Mexico City and Saint Petersburg also fall into this category. City population and especially area are, however, heavily dependent on

388-438: A geomagnetic connection comes from the statistics of auroral observations. Elias Loomis (1860), and later Hermann Fritz (1881) and Sophus Tromholt (1881) in more detail, established that the aurora appeared mainly in the auroral zone. In northern latitudes , the effect is known as the aurora borealis or the northern lights. The southern counterpart, the aurora australis or the southern lights, has features almost identical to

485-401: A geomagnetic storm temporarily enlarges the auroral oval. Large geomagnetic storms are most common during the peak of the 11-year sunspot cycle or during the three years after the peak. An electron spirals (gyrates) about a field line at an angle that is determined by its velocity vectors, parallel and perpendicular, respectively, to the local geomagnetic field vector B. This angle is known as

582-544: A high urbanization level, with an economically specialized city population drawing also on rural resources outside the area, illustrating the difference between high population density and overpopulation . Deserts have very limited potential for growing crops as there is not enough rain to support them. Thus, their population density is generally low. However, some cities in the Middle East, such as Dubai , have been increasing in population and infrastructure growth at

679-408: A little less than the land area of Puerto Rico , 8,868 square kilometres (3,424 sq mi). Although the arithmetic density is the most common way of measuring population density, several other methods have been developed to provide alternative measures of population density over a specific area. Aurora borealis An aurora ( pl. aurorae or auroras ), also commonly known as

776-412: A new, larger Harstad Municipality with a population of 17,882. Prior to the merger, the town of Harstad had 3,808 residents. On 1 January 2013, the neighboring Bjarkøy Municipality (to the north) was merged with Harstad, forming a new, larger Harstad Municipality. On 1 January 2020, the municipality became part of the new Troms og Finnmark county which replaced the old Troms county. On 1 January 2024,

873-1096: A northbound and a southbound Hurtigruten ship stop in Harstad. High-speed craft regularly go between Harstad and Tromsø , Finnsnes , Senja and other places. There are several ferries and buses in the district, and in Harstad there are local buses. The leading helicopter company in Northern Norway, Heli-Team, is located in Harstad. Alvestad , Aune, Elgsnes , Ervik, Grøtavær , Hagan, Kasfjord , Kilhus, Kjøtta, Lundenes , Mustaparta, Nergården , Røkenes, Steinnes , Stornes, Storvassbotn, Sørlia, Tennvassåsen, Tømmeråsen, Undlandet, Vika, and Årnes. Bergseng, Blåbærhaugen, Breivika, City Center , Eineberget, Gangsås , Grønnebakkan , Harstadbotn, Harstadåsen, Heggen , Holtet, Kanebogen , Kilbotn , Medkila , Ruggevika, Sama , Seljestad , Skaret, Stangnes , Trondenes , and Åsby . Brokvik, Fauskevåg , Gausvik , Halsebø, Haukebø, Melvik, Nordvik, Sandtorg , and Sørvika . The twin towns of Harstad are: Harstad used to be

970-432: A particle depends on the particle's pitch angle : the angle between its direction of motion and the local magnetic field. An aurora is created by processes that decrease the pitch angle of many individual electrons, freeing them from the magnetic trap and causing them to hit the atmosphere. In the case of diffuse auroras, the electron pitch angles are altered by their interaction with various plasma waves . Each interaction

1067-469: A sister city to Kirovsk , but this agreement was ended in November 2022, due to Russia's invasion of Ukraine. Population density Population density (in agriculture : standing stock or plant density ) is a measurement of population per unit land area. It is mostly applied to humans , but sometimes to other living organisms too. It is a key geographical term. Population density

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1164-513: A southward component of the IMF, which can then directly connect to the high latitude geomagnetic field lines. The flow pattern of magnetospheric plasma is mainly from the magnetotail toward Earth, around Earth and back into the solar wind through the magnetopause on the day-side. In addition to moving perpendicular to Earth's magnetic field, some magnetospheric plasma travels down along Earth's magnetic field lines, gains additional energy and loses it to

1261-501: A velocity typically around 400 km/s, a density of around 5 ions/cm and a magnetic field intensity of around 2–5 nT (for comparison, Earth's surface field is typically 30,000–50,000 nT). During magnetic storms , in particular, flows can be several times faster; the interplanetary magnetic field (IMF) may also be much stronger. Joan Feynman deduced in the 1970s that the long-term averages of solar wind speed correlated with geomagnetic activity. Her work resulted from data collected by

1358-606: A weak glow (often deep red) observed around the two polar cusps, the field lines separating the ones that close through Earth from those that are swept into the tail and close remotely. Early work on the imaging of the auroras was done in 1949 by the University of Saskatchewan using the SCR-270 radar. The altitudes where auroral emissions occur were revealed by Carl Størmer and his colleagues, who used cameras to triangulate more than 12,000 auroras. They discovered that most of

1455-535: A year. These factors combined can lead to minor cyclical changes in the detailed way that the IMF links to the magnetosphere. In turn, this affects the average probability of opening a door through which energy from the solar wind can reach Earth's inner magnetosphere and thereby enhance auroras. Recent evidence in 2021 has shown that individual separate substorms may in fact be correlated networked communities. Just as there are many types of aurora, there are many different mechanisms that accelerate auroral particles into

1552-684: Is Harstad Idrettslag (a.k.a. HIL), and the most successful basketball team is the Harstad Vikings . Harstad is home port for the Anna Rogde , the world's oldest sailing schooner , also known as the sailing queen of Norway . Harstad Camping is a campsite located in the municipality. The Church of Norway has five parishes ( sokn ) within Harstad Municipality. It is part of the Trondenes prosti ( deanery ) in

1649-661: Is also substantial archeological evidence of a well-developed Iron Age culture in the area, around 200 AD. Trondenes is mentioned in the Heimskringla as a power centre in the Viking Age and a place to meet and discuss important issues ( Trondarting ). In 2020 archeologist concluded that Sandtorg , located along the Tjeldsundet strait, south of the town of Harstad, was the location of the only known Viking Age trading place in Northern Norway. The Tjeldsundet strait

1746-530: Is bordered by Kvæfjord Municipality to the west. To the south and east, the Tjeldsund Bridge connects Hinnøya with Tjeldsund Municipality and the mainland across Tjeldsundet , and to the northeast is the Vågsfjorden , where Harstad shares a water border with Ibestad Municipality . The city of Harstad itself is located on the northeastern part of Hinnøya; it is the only city on the island, and

1843-574: Is calculated for a county , city , country , another territory or the entire world . The world's population is around 8,000,000,000 and the Earth 's total area (including land and water) is 510,000,000 km (200,000,000 sq mi). Therefore, the worldwide human population density is approximately 8,000,000,000 ÷ 510,000,000 = 16/km (41/sq mi). However, if only the Earth's land area of 150,000,000 km (58,000,000 sq mi)

1940-443: Is essentially wave-particle scattering ; the electron energy after interacting with the wave is similar to its energy before interaction, but the direction of motion is altered. If the final direction of motion after scattering is close to the field line (specifically, if it falls within the loss cone ) then the electron will hit the atmosphere. Diffuse auroras are caused by the collective effect of many such scattered electrons hitting

2037-444: Is generally accompanied by decreasing peak emission heights of about 8 km for blue and green emissions and above average solar wind speeds ( c.  500   km/s ). In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as auroral kilometric radiation (AKR), discovered in 1972. Ionospheric absorption makes AKR only observable from space. X-ray emissions, originating from

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2134-573: Is governed by a municipal council of directly elected representatives. The mayor is indirectly elected by a vote of the municipal council. The municipality is under the jurisdiction of the Midtre Hålogaland District Court and the Hålogaland Court of Appeal . The municipal council (Kommunestyre) of Harstad is made up of 35 representatives that are elected to four-year terms. The tables below show

2231-520: Is induced within the conductor. The strength of the current depends on a) the rate of relative motion, b) the strength of the magnetic field, c) the number of conductors ganged together and d) the distance between the conductor and the magnetic field, while the direction of flow is dependent upon the direction of relative motion. Dynamos make use of this basic process ("the dynamo effect "), any and all conductors, solid or otherwise are so affected, including plasmas and other fluids. The IMF originates on

2328-484: Is more effectively transferred by the temporary magnetic connection between the field lines of the solar wind and those of the magnetosphere. Unsurprisingly this process is known as magnetic reconnection . As already mentioned, it happens most readily when the interplanetary field is directed southward, in a similar direction to the geomagnetic field in the inner regions of both the north magnetic pole and south magnetic pole . Auroras are more frequent and brighter during

2425-423: Is popularly known as Vågsfjordens perle (The pearl of Vågsfjorden). The highest point in Harstad Municipality is Sætertinden, which is 1,095 m (3,593 ft) above sea level. It is located near the village of Sandtorg in southern Harstad. The 412-metre (1,352 ft) tall mountain, Nupen , is located in the northwestern part of the municipality on the border with Kvæfjord. Despite being located north of

2522-592: Is population divided by total land area , sometimes including seas and oceans, as appropriate. Low densities may cause an extinction vortex and further reduce fertility. This is called the Allee effect after the scientist who identified it. Examples of the causes of reduced fertility in low population densities are: Population density is the number of people per unit of area, usually transcribed as "per square kilometer" or square mile, and which may include or exclude, for example, areas of water or glaciers. Commonly this

2619-407: Is regularly derived from ground data and serves as a general measure of auroral activity. Kristian Birkeland deduced that the currents flowed in the east–west directions along the auroral arc, and such currents, flowing from the dayside toward (approximately) midnight were later named "auroral electrojets" (see also Birkeland currents ). Ionosphere can contribute to the formation of auroral arcs via

2716-530: Is taken into account, then human population density is 53/km (140/sq mi). This includes all continental and island land area, including Antarctica . However, if Antarctica is excluded, then population density rises to over 58 per square kilometre (150/sq mi). The European Commission's Joint Research Centre (JRC) has developed a suite of (open and free) data and tools named the Global Human Settlement Layer (GHSL) to improve

2813-711: Is well-suited for farming. The city hosts the annual week-long Festival of North Norway in June. It is also the home of the Arctic Moving Image and Film Festival, held in October each year. Harstad University College , with approximately 1,100 students, has a thriving foreign exchange program with students from all over the world. The hospital in Harstad is part of the University Hospital of North Norway . The most successful local football team

2910-522: Is why there is a color differential with altitude; at high altitudes oxygen red dominates, then oxygen green and nitrogen blue/purple/red, then finally nitrogen blue/purple/red when collisions prevent oxygen from emitting anything. Green is the most common colour. Then comes pink, a mixture of light green and red, followed by pure red, then yellow (a mixture of red and green), and finally, pure blue. Precipitating protons generally produce optical emissions as incident hydrogen atoms after gaining electrons from

3007-488: The Arctic Circle , Harstad features either a dry-summer subarctic climate ( Köppen climate classification : Dsc ), or (barely, by only half a degree C) the rare cold-summer mediterranean climate ( Köppen climate classification : Csc ), depending on if the 0 °C (32 °F) or the −3 °C (27 °F) isotherm is used. Harstad features relatively mild, wet winters and cool, dry summers. Harstad does not have

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3104-540: The DNV office for Northern Norway, as well as other regional offices including TotalEnergies , Det Norske Oljeselskap ASA and Aibel . Harstad also has shipyards and other industries that are important for the economy. Harstad and the surrounding area have traditionally been among the most productive agricultural regions in Northern Norway,. The old seabed, now dry land due to isostatic rebound (up to 60 to 80 metres or 200 to 260 feet above sea level), creating fertile soil that

3201-596: The Diocese of Nord-Hålogaland . Harstad traditionally has strong ties with the Norwegian Armed Forces. Kystjegerkommandoen (Coastal Ranger Command) has its home base at Trondenes , just north of the town of Harstad. Marinejegerkommandoen is based in Ramsund , a village in neighboring Tjeldsund Municipality on the mainland south of Harstad. General Carl Gustav Fleischer led the field operations of

3298-481: The Explorer 33 spacecraft. The solar wind and magnetosphere consist of plasma (ionized gas), which conducts electricity. It is well known (since Michael Faraday 's work around 1830) that when an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cuts across (or is cut by ), rather than along , the lines of the magnetic field, an electric current

3395-502: The Nazis . Harstad is one of the few towns in this part of Norway which were left largely undamaged by World War II. The municipality (and town ) is named after the old Harstad farm ( Old Norse : Harðarstaðir ), since the town is built where the farm once was located. The first element is (probably) the genitive case of the male name Hǫrðr . The last element is staðir which means " homestead " or " farm ". On 6 February 2017,

3492-519: The Troms og Finnmark county was divided and the municipality once again became part of Troms county. In recent years, a 3000-year-old bronze axe and a 2600-year-old bronze collar have been found at the Trondenes peninsula, just north of the city center. These, together with the burial cairns built close to the sea, are indications of a well-developed Bronze Age culture in the Harstad area. There

3589-467: The feedback instability under high ionospheric resistance conditions, observed at night time and in dark Winter hemisphere. Earth is constantly immersed in the solar wind , a flow of magnetized hot plasma (a gas of free electrons and positive ions) emitted by the Sun in all directions, a result of the two-million-degree temperature of the Sun's outermost layer, the corona . The solar wind reaches Earth with

3686-411: The mesosphere in presence of electron precipitation . Horse-collar auroras (HCA) are auroral features in which the auroral ellipse shifts poleward during the dawn and dusk portions and the polar cap becomes teardrop-shaped. They form during periods when the interplanetary magnetic field (IMF) is permanently northward, when the IMF clock angle is small. Their formation is associated with the closure of

3783-488: The northern lights ( aurora borealis ) or southern lights ( aurora australis ), is a natural light display in Earth 's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic ). Auroras display dynamic patterns of brilliant lights that appear as curtains, rays, spirals, or dynamic flickers covering the entire sky. Auroras are the result of disturbances in the Earth's magnetosphere caused by

3880-488: The solar wind . Major disturbances result from enhancements in the speed of the solar wind from coronal holes and coronal mass ejections . These disturbances alter the trajectories of charged particles in the magnetospheric plasma . These particles, mainly electrons and protons , precipitate into the upper atmosphere ( thermosphere / exosphere ). The resulting ionization and excitation of atmospheric constituents emit light of varying colour and complexity. The form of

3977-455: The "pitch angle" of the particle. The distance, or radius, of the electron from the field line at any time is known as its Larmor radius. The pitch angle increases as the electron travels to a region of greater field strength nearer to the atmosphere. Thus, it is possible for some particles to return, or mirror, if the angle becomes 90° before entering the atmosphere to collide with the denser molecules there. Other particles that do not mirror enter

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4074-409: The 1960s by many research teams using rockets and satellites to traverse the auroral zone. The main findings have been that auroral arcs and other bright forms are due to electrons that have been accelerated during the final few 10,000 km or so of their plunge into the atmosphere. These electrons often, but not always, exhibit a peak in their energy distribution, and are preferentially aligned along

4171-580: The 22 °C (72 °F) mark. Since the weather station opened August 2002, July 2014 was the warmest month with mean 16.9 °C (62 °F), average daily high 21.8 °C (71 °F) and all-time high 31.7 °C (89 °F) on 10 July. The record low of −16.1 °C (3 °F) was recorded in February 2010. The coldest month recorded was January 2003 with mean −6.5 °C (20 °F) and average daily low −9.5 °C (15 °F). On 9. September 2024 Harstad recorded high 25.9 °C (79 °F),

4268-519: The Harstad district, sinking two ships and a U-boat. Harstad is also the hometown of the Norwegian army band "Forsvarets Musikkorps Nord Norge" with professional musicians. The local airport is Harstad/Narvik Airport, Evenes , located on the mainland in neighboring Evenes Municipality . It is 44 kilometres (27 mi) by road from the town center. The airport offers daily flights to Oslo , Trondheim , Bodø , Tromsø and Andenes . Every morning

4365-649: The Norwegian Armed Forces in World War II , among them the 7,500 soldiers which from the north pushed the Nazi Germans back to Narvik and participated in retaking Narvik on 28 May 1940. A street in Harstad is named Gen. Fleischers Gate in his honour. Operation Judgement, Kilbotn took place on 4 May 1945, when the Fleet Air Arm of the Royal Navy attacked a U-boat base at Kilbotn, a village in

4462-524: The Sun, linked to the sunspots , and its field lines (lines of force) are dragged out by the solar wind. That alone would tend to line them up in the Sun-Earth direction, but the rotation of the Sun angles them at Earth by about 45 degrees forming a spiral in the ecliptic plane, known as the Parker spiral . The field lines passing Earth are therefore usually linked to those near the western edge ("limb") of

4559-399: The arms have a blue field (background) and the charge is two wavy bars . The bars have a tincture of argent which means they are commonly colored white, but if it is made out of metal, then silver is used. The blue color in the field symbolizes the importance of sea and the wavy bars were chosen to represent the waves in the sea. Since the town of Harstad is located in the municipality,

4656-486: The atmosphere and contribute to the auroral display over a range of altitudes. Other types of auroras have been observed from space; for example, "poleward arcs" stretching sunward across the polar cap, the related "theta aurora", and "dayside arcs" near noon. These are relatively infrequent and poorly understood. Other interesting effects occur such as pulsating aurora, "black aurora" and their rarer companion "anti-black aurora" and subvisual red arcs. In addition to all these,

4753-423: The atmosphere in the auroral zones. The cusps of the magnetosphere, separating geomagnetic field lines that close through Earth from those that close remotely allow a small amount of solar wind to directly reach the top of the atmosphere, producing an auroral glow. On 26 February 2008, THEMIS probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms . Two of

4850-456: The atmosphere. Electron aurora in Earth's auroral zone (i.e. commonly visible aurora) can be split into two main categories with different immediate causes: diffuse and discrete aurora. Diffuse aurora appear relatively structureless to an observer on the ground, with indistinct edges and amorphous forms. Discrete aurora are structured into distinct features with well-defined edges such as arcs, rays and coronas; they also tend to be much brighter than

4947-447: The atmosphere. Proton auroras are usually observed at lower latitudes. Bright auroras are generally associated with Birkeland currents (Schield et al., 1969; Zmuda and Armstrong, 1973 ), which flow down into the ionosphere on one side of the pole and out on the other. In between, some of the current connects directly through the ionospheric E layer (125 km); the rest ("region 2") detours, leaving again through field lines closer to

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5044-439: The atmosphere. The process is mediated by the plasma waves, which become stronger during periods of high geomagnetic activity , leading to increased diffuse aurora at those times. In the case of discrete auroras, the trapped electrons are accelerated toward Earth by electric fields that form at an altitude of about 4000–12000 km in the "auroral acceleration region". The electric fields point away from Earth (i.e. upward) along

5141-732: The aurora borealis and changes simultaneously with changes in the northern auroral zone. The aurora australis is visible from high southern latitudes in Antarctica , the Southern Cone , South Africa , Australasia and under exceptional circumstances as far north as Uruguay . The aurora borealis is visible from areas around the Arctic such as Alaska , Canada , Iceland , Greenland , the Faroe Islands , Scandinavia , Finland , Scotland , and Russia . A geomagnetic storm causes

5238-599: The aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles. Most of the planets in the Solar System , some natural satellites , brown dwarfs , and even comets also host auroras. The term aurora borealis was coined by Galileo in 1619, from the Roman Aurora, goddess of the dawn , and the Greek Boreas, god of

5335-518: The auroral oval to active displays along the darkside and after 1–3 hours they gradually change back. Changes in auroras over time are commonly visualized using keograms . At shorter time scales, auroras can change their appearances and intensity, sometimes so slowly as to be difficult to notice, and at other times rapidly down to the sub-second scale. The phenomenon of pulsating auroras is an example of intensity variations over short timescales, typically with periods of 2–20 seconds. This type of aurora

5432-726: The auroral ovals (north and south) to expand, bringing the aurora to lower latitudes. On rare occasions, the aurora borealis can be seen as far south as the Mediterranean and the southern states of the US while the aurora australis can be seen as far north as New Caledonia and the Pilbara region in Western Australia . During the Carrington Event , the greatest geomagnetic storm ever observed, auroras were seen even in

5529-412: The background magnetic field (comparable to the electron inertial length or ion gyroradius ), Alfvén waves develop a significant electric field parallel to the background magnetic field. This electric field can accelerate electrons to keV energies, significant to produce auroral arcs. If the electrons have a speed close to that of the wave's phase velocity, they are accelerated in a manner analogous to

5626-558: The brutal winters most locations north of the Arctic Circle experience, and is sheltered from Atlantic gales by mountains in the west, and has the main part of the Scandinavian Mountains to the east. The city experiences warmer winters than major cities located 25 degrees farther south in latitude such as Beijing , Chicago and Toronto . Summers in Harstad are cool, with average high temperatures seldom breaking

5723-524: The cold north wind . The word aurora is derived from the name of the Roman goddess of the dawn, Aurora , who travelled from east to west announcing the coming of the Sun . Ancient Greek poets used the corresponding name " Eos " metaphorically to refer to dawn, often mentioning its play of colours across the otherwise dark sky (e.g., "rosy-fingered dawn"). The words borealis and australis are derived from

5820-465: The collision of particles precipitated into the atmosphere. Both incoming electrons and protons may be involved. Excitation energy is lost within the atmosphere by the emission of a photon, or by collision with another atom or molecule: Oxygen is unusual in terms of its return to ground state: it can take 0.7 seconds to emit the 557.7 nm green light and up to two minutes for the red 630.0 nm emission. Collisions with other atoms or molecules absorb

5917-413: The current and historical composition of the council by political party . The mayor ( Norwegian : ordfører ) of Harstad is the political leader of the municipality and the chairperson of the municipal council. Here is a list of people who have held this position: The oil industry of North Norway is centered in Harstad; including Statoil 's main office for a new operational area for Northern Norway,

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6014-468: The currents are the direct result of electron acceleration into the atmosphere by wave/particle interactions. Ionospheric resistance has a complex nature, and leads to a secondary Hall current flow. By a strange twist of physics, the magnetic disturbance on the ground due to the main current almost cancels out, so most of the observed effect of auroras is due to a secondary current, the auroral electrojet . An auroral electrojet index (measured in nanotesla)

6111-547: The definition of "urban area" used: densities are almost invariably higher for the center only than when suburban settlements and intervening rural areas are included, as in the agglomeration or metropolitan area (the latter sometimes including neighboring cities). In comparison, based on a world population of 8 billion, the world's inhabitants, if conceptualized as a loose crowd occupying just under 1 m (10 sq ft) per person (cf. Jacobs Method ), would occupy an area of 8,000 square kilometres (3,100 sq mi)

6208-416: The diffuse aurora. In both cases, the electrons that eventually cause the aurora start out as electrons trapped by the magnetic field in Earth's magnetosphere . These trapped particles bounce back and forth along magnetic field lines and are prevented from hitting the atmosphere by the magnetic mirror formed by the increasing magnetic field strength closer to Earth. The magnetic mirror's ability to trap

6305-404: The dune aurora phenomenon was discovered by Finnish citizen scientists . It consists of regularly-spaced, parallel stripes of brighter emission in the green diffuse aurora which give the impression of sand dunes. The phenomenon is believed to be caused by the modulation of atomic oxygen density by a large-scale atmospheric wave travelling horizontally in a waveguide through an inversion layer in

6402-458: The electric field increases the kinetic energy of all of the electrons transiting downward through the acceleration region by the same amount. This accelerates electrons starting from the magnetosphere with initially low energies (tens of eV or less) to energies required to create an aurora (100s of eV or greater), allowing that large source of particles to contribute to creating auroral light. The accelerated electrons carry an electric current along

6499-454: The equator and closing through the "partial ring current" carried by magnetically trapped plasma. The ionosphere is an ohmic conductor , so some consider that such currents require a driving voltage, which an, as yet unspecified, dynamo mechanism can supply. Electric field probes in orbit above the polar cap suggest voltages of the order of 40,000 volts, rising up to more than 200,000 volts during intense magnetic storms. In another interpretation,

6596-490: The excitation energy and prevent emission; this process is called collisional quenching . Because the highest parts of the atmosphere contain a higher percentage of oxygen and lower particle densities, such collisions are rare enough to allow time for oxygen to emit red light. Collisions become more frequent progressing down into the atmosphere due to increasing density, so that red emissions do not have time to happen, and eventually, even green light emissions are prevented. This

6693-465: The five probes, positioned approximately one third the distance to the Moon, measured events suggesting a magnetic reconnection event 96 seconds prior to auroral intensification. Geomagnetic storms that ignite auroras may occur more often during the months around the equinoxes . It is not well understood, but geomagnetic storms may vary with Earth's seasons. Two factors to consider are the tilt of both

6790-447: The geomagnetic pole (not the geographic pole) in the noon direction and 23° away in the midnight direction. The peak equatorward extent of the oval is displaced slightly from geographic midnight. It is centered about 3–5° nightward of the magnetic pole, so that auroral arcs reach furthest toward the equator when the magnetic pole in question is in between the observer and the Sun , which is called magnetic midnight . Early evidence for

6887-430: The intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind. Earth's magnetosphere is shaped by the impact of the solar wind on Earth's magnetic field. This forms an obstacle to the flow, diverting it, at an average distance of about 70,000 km (11 Earth radii or Re), producing a bow shock 12,000 km to 15,000 km (1.9 to 2.4 Re) further upstream. The width of

6984-401: The interaction of the solar wind with Earth's magnetosphere . The varying intensity of the solar wind produces effects of different magnitudes but includes one or more of the following physical scenarios. The details of these phenomena are not fully understood. However, it is clear that the prime source of auroral particles is the solar wind feeding the magnetosphere, the reservoir containing

7081-509: The light is produced between 90 and 150 km (56 and 93 mi) above the ground, while extending at times to more than 1,000 km (620 mi). According to Clark (2007), there are five main forms that can be seen from the ground, from least to most visible: Brekke (1994) also described some auroras as "curtains". The similarity to curtains is often enhanced by folds within the arcs. Arcs can fragment or break up into separate, at times rapidly changing, often rayed features that may fill

7178-762: The local direction of the magnetic field. Electrons mainly responsible for diffuse and pulsating auroras have, in contrast, a smoothly falling energy distribution, and an angular (pitch-angle) distribution favouring directions perpendicular to the local magnetic field. Pulsations were discovered to originate at or close to the equatorial crossing point of auroral zone magnetic field lines. Protons are also associated with auroras, both discrete and diffuse. Auroras result from emissions of photons in Earth's upper atmosphere , above 80 km (50 mi), from ionized nitrogen atoms regaining an electron, and oxygen atoms and nitrogen based molecules returning from an excited state to ground state . They are ionized or excited by

7275-426: The magnetic field line. Electrons moving downward through these fields gain a substantial amount of energy (on the order of a few keV ) in the direction along the magnetic field line toward Earth. This field-aligned acceleration decreases the pitch angle for all of the electrons passing through the region, causing many of them to hit the upper atmosphere. In contrast to the scattering process leading to diffuse auroras,

7372-432: The magnetic field lines (a Birkeland current ). Since the electric field points in the same direction as the current, there is a net conversion of electromagnetic energy into particle energy in the auroral acceleration region (an electric load ). The energy to power this load is eventually supplied by the magnetized solar wind flowing around the obstacle of Earth's magnetic field, although exactly how that power flows through

7469-422: The magnetic flux at the top of the dayside magnetosphere by the double lobe reconnection (DLR). There are approximately 8 HCA events per month, with no seasonal dependence, and that the IMF must be within 30 degrees of northwards. Conjugate auroras are nearly exact mirror-image auroras found at conjugate points in the northern and southern hemispheres on the same geomagnetic field lines. These generally happen at

7566-413: The magnetosphere abreast of Earth is typically 190,000 km (30 Re), and on the night side a long "magnetotail" of stretched field lines extends to great distances (> 200 Re). The high latitude magnetosphere is filled with plasma as the solar wind passes Earth. The flow of plasma into the magnetosphere increases with additional turbulence, density, and speed in the solar wind. This flow is favoured by

7663-490: The magnetosphere is still an active area of research. While the energy to power the aurora is ultimately derived from the solar wind, the electrons themselves do not travel directly from the solar wind into Earth's auroral zone; magnetic field lines from these regions do not connect to the solar wind, so there is no direct access for solar wind electrons. Some auroral features are also created by electrons accelerated by dispersive Alfvén waves . At small wavelengths transverse to

7760-711: The municipality is located on the large island of Hinnøya , which is Norway's largest coastal island (three islands in the Svalbard archipelago are larger). The northern part of the municipality is located on the smaller islands of Grytøya , Bjarkøya , Sandsøya , Helløya, Flatøya and Krøttøya and many even smaller islands between the Andfjorden (to the west) and the Vågsfjorden (to the east). The municipality contains several small islands, including Arnøya, Gressholman , Helløya, Kjeøya, Kjøtta, Kjøttakalven, Flatøya, Meløyvær , Måga, Rogla, Lille Rogla and Åkerøya. Harstad

7857-554: The municipality of Harstad adopted a co-equal Sami language name for the municipality: Hárstták. The Sami language name spelling changes depending on how it is used. It is called Hárstták when it is spelled alone, but it is Hársttáid suohkan when using the Sami language equivalent to "Harstad municipality". The coat of arms was granted on 24 April 1953. The official blazon is " Azure , two bars wavy argent " ( Norwegian : To bølgende sølv bjelker på blå bunn ). This means

7954-473: The names of the ancient gods of the north wind ( Boreas ) and the south wind ( Auster ) in Greco-Roman mythology . Auroras are most commonly observed in the "auroral zone", a band approximately 6° (~660 km) wide in latitude centered on 67° north and south. The region that currently displays an aurora is called the "auroral oval". The oval is displaced by the solar wind, pushing it about 15° away from

8051-510: The opposite order. Until about 1963, it was thought that these changes are due to the rotation of the Earth under a pattern fixed with respect to the Sun. Later, it was found by comparing all-sky films of auroras from different places (collected during the International Geophysical Year ) that they often undergo global changes in a process called auroral substorm . They change in a few minutes from quiet arcs all along

8148-456: The particles associated with auroras, have also been detected. Aurora noise , similar to a crackling noise, begins about 70 m (230 ft) above Earth's surface and is caused by charged particles in an inversion layer of the atmosphere formed during a cold night. The charged particles discharge when particles from the Sun hit the inversion layer, creating the noise. In 2016, more than fifty citizen science observations described what

8245-593: The radiation zones and temporarily magnetically trapped particles confined by the geomagnetic field, coupled with particle acceleration processes. The immediate cause of the ionization and excitation of atmospheric constituents leading to auroral emissions was discovered in 1960, when a pioneering rocket flight from Fort Churchill in Canada revealed a flux of electrons entering the atmosphere from above. Since then an extensive collection of measurements has been acquired painstakingly and with steadily improving resolution since

8342-431: The ribbon). The processes that cause STEVE are also associated with a picket-fence aurora, although the latter can be seen without STEVE. It is an aurora because it is caused by precipitation of electrons in the atmosphere but it appears outside the auroral oval, closer to the equator than typical auroras. When the picket-fence aurora appears with STEVE, it is below. First reported in 2020, and confirmed in 2021,

8439-568: The science for policy support to the European Commission Directorate Generals and Services and as support to the United Nations system. Several of the most densely populated territories in the world are city-states , microstates and urban dependencies . In fact, 95% of the world's population is concentrated on just 10% of the world's land. These territories have a relatively small area and

8536-416: The solar and Earth's axis to the ecliptic plane. As Earth orbits throughout a year, it experiences an interplanetary magnetic field (IMF) from different latitudes of the Sun, which is tilted at 8 degrees. Similarly, the 23-degree tilt of Earth's axis about which the geomagnetic pole rotates with a diurnal variation changes the daily average angle that the geomagnetic field presents to the incident IMF throughout

8633-455: The south. From late January, the period of daylight rapidly increases, reaching 12 hours by March and 18 hours in April. Harstad is located in the midst of the aurora borealis ( a.k.a. the northern lights ) zone, and the aurora can often be seen on clear nights, but not in summer due to the continuous daylight. The town of Harstad was established as a municipality on 1 January 1904 when it

8730-409: The time of the equinoxes , when there is little difference in the orientation of the north and south geomagnetic poles to the sun. Attempts were made to image conjugate auroras by aircraft from Alaska and New Zealand in 1967, 1968, 1970, and 1971, with some success. A full understanding of the physical processes which lead to different types of auroras is still incomplete, but the basic cause involves

8827-414: The tropics. Auroras seen within the auroral oval may be directly overhead. From farther away, they illuminate the poleward horizon as a greenish glow, or sometimes a faint red, as if the Sun were rising from an unusual direction. Auroras also occur poleward of the auroral zone as either diffuse patches or arcs, which can be subvisual. Auroras are occasionally seen in latitudes below the auroral zone, when

8924-406: The visible Sun at any time. The solar wind and the magnetosphere, being two electrically conducting fluids in relative motion, should be able in principle to generate electric currents by dynamo action and impart energy from the flow of the solar wind. However, this process is hampered by the fact that plasmas conduct readily along magnetic field lines, but less readily perpendicular to them. Energy

9021-621: The warmest temperature in September recorded in Troms. The municipality enjoys the midnight sun during the summer months, from 22 May to 18 July. There is also a period from early May to early August with twilight for a few hours each night as the sun just dips below the horizon, so there is no darkness. The polar night , when the sun is always below the horizon, lasts from 30 November to 12 January. At this time, there are 3–4 hours of dawn and dusk around noon, sometimes with colourful skies towards

9118-506: The whole sky. These are also known as discrete auroras , which are at times bright enough to read a newspaper by at night. These forms are consistent with auroras being shaped by Earth's magnetic field. The appearances of arcs, rays, curtains, and coronas are determined by the shapes of the luminous parts of the atmosphere and a viewer's position . Auroras change with time. Over the night they begin with glows and progress toward coronas, although they may not reach them. They tend to fade in

9215-534: Was separated from the large Trondenes Municipality because it had just been declared a ladested (small seaport). The initial population of the town of Harstad was 1,246. During the 1960s, there were many municipal mergers across Norway due to the work of the Schei Committee . On 1 January 1964, the town of Harstad (population: 3,808) was merged with neighboring Sandtorg Municipality (population: 7,512) and Trondenes Municipality (population: 6,567) to form

9312-424: Was to them an unknown type of aurora which they named " STEVE ", for "Strong Thermal Emission Velocity Enhancement". STEVE is not an aurora but is caused by a 25 km (16 mi) wide ribbon of hot plasma at an altitude of 450 km (280 mi), with a temperature of 3,000 °C (3,270 K; 5,430 °F) and flowing at a speed of 6 km/s (3.7 mi/s) (compared to 10 m/s (33 ft/s) outside

9409-547: Was very likely an important ship lane back then as it still is today. Trondenes Church , the world's northernmost medieval church, which dates back to the 13th–15th century, is situated just outside the town. Adjacent to the church is the Trondenes Historical Center and nearby is the Adolf Gun , an enormous land-based cannon from World War II , and the last of four cannons originally constructed by

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