The equatorial electrojet (EEJ) is a narrow ribbon of current flowing eastward in the day time equatorial region of the Earth's ionosphere. The abnormally large amplitude of variations in the horizontal components measured at equatorial geomagnetic observatories, as a result of EEJ, was noticed as early as 1920 from Huancayo geomagnetic observatory. Observations by radar, rockets, satellites, and geomagnetic observatories are used to study EEJ.
The explanation [1] for the existence of the equatorial electrojet lies with the anisotropic nature of ionospheric electrical conductivity and a process of self-reinforcement. Global-scale ionospheric circulation establishes a Sq (solar quiet) current system in the E region of the Earth's ionosphere (100–130 km altitude), and a primary eastwards electric field near day-side magnetic equator, where the magnetic field is horizontal and northwards. This electric field gives a primary eastwards Pedersen current. E cross B drift results in a downwards Hall current, sustaining vertical charge separation across the depth of the ionosphere, giving an upwards secondary electric field and a secondary Pedersen current that is opposite to the primary Hall current. A secondary Hall current then reinforces the original Pedersen current. At about 110 km height, the integration of the current density gives a peak current strength of about 100 kA, which supports a day-side electrojet magnetic-field enhancement of a factor of two or so.
As the position of the sun, moon, and earth changes, so does the strength of the lunar tidal forces. Each lunar month, two spring tides occur when the sun, moon, and earth are aligned to produce a strong lunar tidal force. Likewise, two neap tides occur when the sun and moon are adjacent to one another to produce weak lunar tidal forces. The equatorial electrojet (EEJ) has an abnormally large amplitude of variations in the horizontal components due to the strength of the lunar tides. The lunar tide varies as described above and is changed by the gravitational attraction between the Moon and Earth. Because of this, the pressure and temperature of the lower atmosphere vary, and the effects propagate upward in a tidal wave form to the E region and modulate the electrodynamics.
The EEJ phenomenon was first identified using geomagnetic data. The amplitude of the daily variation of the horizontal magnetic intensity (ΔH) measured at a geomagnetic observatory near the dip-equator is 3–5 fold higher than the variation of data from other regions of Earth. A typical diurnal equatorial observatory data show a peak of strength ~80 nT at 12:00 LT, with respect to the night-time level. Egedal (1947) showed that the enhancement of quiet day solar daily variations in ΔH (Sq(H)) lay within the 50 latitude centered on the dip equator. The mechanism that produced the variation in the magnetic field was proposed as a band of current about 300 km in width flowing over the dip equator.
EEJ studies from satellite data were initiated with the arrival of data from the POGO (Polar Orbiting Geophysical Observatories) series of satellite (1967–1970). The characteristic signature of the EEJ is a sharp negative V-shaped curve in the H field, attaining its minimum within 0.5° of the magnetic dip equator. The magnetic data from satellite missions like Ørsted (1999–present) and CHAMP (2000–present) have vastly improved our knowledge of the EEJ.
Recent studies have focused on the lunar-solar interaction is the EEJ. It was demonstrated that complexity is introduced into the EEJ due to the interaction between lunar tide variability in the equatorial electric field and solar-driven variability in the E-region conductivity. [2]
The ionosphere is the ionized part of the upper atmosphere of Earth, from about 48 km (30 mi) to 965 km (600 mi) above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on Earth. It also affects GPS signals that travel through this layer.
The magnetopause is the abrupt boundary between a magnetosphere and the surrounding plasma. For planetary science, the magnetopause is the boundary between the planet's magnetic field and the solar wind. The location of the magnetopause is determined by the balance between the pressure of the dynamic planetary magnetic field and the dynamic pressure of the solar wind. As the solar wind pressure increases and decreases, the magnetopause moves inward and outward in response. Waves along the magnetopause move in the direction of the solar wind flow in response to small-scale variations in the solar wind pressure and to Kelvin–Helmholtz instability.
The thermosphere is the layer in the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the thermosphere thus constitutes the larger part of the ionosphere. Taking its name from the Greek θερμός meaning heat, the thermosphere begins at about 80 km (50 mi) above sea level. At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation. Temperatures are highly dependent on solar activity, and can rise to 2,000 °C (3,630 °F) or more. Radiation causes the atmospheric particles in this layer to become electrically charged, enabling radio waves to be refracted and thus be received beyond the horizon. In the exosphere, beginning at about 600 km (375 mi) above sea level, the atmosphere turns into space, although, by the judging criteria set for the definition of the Kármán line (100 km), most of the thermosphere is part of space. The border between the thermosphere and exosphere is known as the thermopause.
An aurora , also commonly known as the northern lights or southern lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions. Auroras display dynamic patterns of brilliant lights that appear as curtains, rays, spirals, or dynamic flickers covering the entire sky.
Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the varying conditions within the Solar System and its heliosphere. This includes the effects of the solar wind, especially in the Earth's magnetosphere, ionosphere, thermosphere, and exosphere. Though physically distinct, space weather is analogous to the terrestrial weather of Earth's atmosphere. The term "space weather" was first used in the 1950s and popularized in the 1990s. Later, it prompted research into "space climate", the large-scale and long-term patterns of space weather.
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic field is generated by electric currents due to the motion of convection currents of a mixture of molten iron and nickel in Earth's outer core: these convection currents are caused by heat escaping from the core, a natural process called a geodynamo.
A geomagnetic storm, also known as a magnetic storm, is temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave.
A whistler is a very low frequency (VLF) electromagnetic (radio) wave generated by lightning. Frequencies of terrestrial whistlers are 1 kHz to 30 kHz, with maximum frequencies usually at 3 kHz to 5 kHz. Although they are electromagnetic waves, they occur at audio frequencies, and can be converted to audio using a suitable receiver. They are produced by lightning strikes where the impulse travels along the Earth's magnetic field lines from one hemisphere to the other. They undergo dispersion of several kHz due to the slower velocity of the lower frequencies through the plasma environments of the ionosphere and magnetosphere. Thus they are perceived as a descending tone which can last for a few seconds. The study of whistlers categorizes them into Pure Note, Diffuse, 2-Hop, and Echo Train types.
The Kodaikanal Solar Observatory is a solar observatory owned and operated by the Indian Institute of Astrophysics. It is on the southern tip of the Palani Hills 4 kilometres (2.5 mi) from Kodaikanal.
A Birkeland current is a set of electrical currents that flow along geomagnetic field lines connecting the Earth's magnetosphere to the Earth's high latitude ionosphere. In the Earth's magnetosphere, the currents are driven by the solar wind and interplanetary magnetic field and by bulk motions of plasma through the magnetosphere. The strength of the Birkeland currents changes with activity in the magnetosphere. Small scale variations in the upward current sheets accelerate magnetospheric electrons which, when they reach the upper atmosphere, create the Auroras Borealis and Australis. In the high latitude ionosphere, the Birkeland currents close through the region of the auroral electrojet, which flows perpendicular to the local magnetic field in the ionosphere. The Birkeland currents occur in two pairs of field-aligned current sheets. One pair extends from noon through the dusk sector to the midnight sector. The other pair extends from noon through the dawn sector to the midnight sector. The sheet on the high latitude side of the auroral zone is referred to as the Region 1 current sheet and the sheet on the low latitude side is referred to as the Region 2 current sheet.
An electrojet is an electric current which travels around the E region of the Earth's ionosphere. There are three electrojets: one above the magnetic equator, and one each near the Northern and Southern Polar Circles. Electrojets are Hall currents carried primarily by electrons at altitudes from 100 to 150 km. In this region the electron gyro frequency is much greater than the electron-neutral collision frequency. In contrast, the principal E region ions have gyrofrequencies much lower than the ion-neutral collision frequency.
The K-index quantifies disturbances in the horizontal component of Earth's magnetic field with an integer in the range 0–9 with 1 being calm and 5 or more indicating a geomagnetic storm. It is derived from the maximum fluctuations of horizontal components observed on a magnetometer during a three-hour interval. The label K comes from the German word Kennziffer meaning "characteristic digit". The K-index was introduced by Julius Bartels in 1939.
Geomagnetically induced currents (GIC) are electrical currents induced at the Earth's surface by rapid changes in the geomagnetic field caused by space weather events. GICs can affect the normal operation of long electrical conductor systems such as electric transmission grids and buried pipelines. The geomagnetic disturbances which induce GICs include geomagnetic storms and substorms where the most severe disturbances occur at high geomagnetic latitudes.
Colaba Observatory, also known as the Bombay Observatory, was an astronomical, timekeeping, geomagnetic and meteorological observatory located on the Island of Colaba, Mumbai (Bombay), India.
The Jicamarca Radio Observatory (JRO) is the equatorial anchor of the Western Hemisphere chain of Incoherent Scatter Radar (ISR) observatories extending from Lima, Peru to Søndre Strømfjord, Greenland. JRO is the premier scientific facility in the world for studying the equatorial ionosphere. The observatory is about half an hour drive inland (east) from Lima and 10 km from the Central Highway. The magnetic dip angle is about 1°, and varies slightly with altitude and year. The radar can accurately determine the direction of the Earth's magnetic field (B) and can be pointed perpendicular to B at altitudes throughout the ionosphere. The study of the equatorial ionosphere is rapidly becoming a mature field due, in large part, to the contributions made by JRO in radio science.
Satya Prakash is an Indian plasma physicist and a former senior professor at the Physical Research Laboratory. He is known for his studies on Langmuir probes and other contributions in space and plasma sciences. A protégé of Vikram Sarabhai, Satya Prakash is an elected fellow of all the three major Indian science academies such as Indian Academy of Sciences, Indian National Science Academy and National Academy of Sciences, India as well as the Gujarat Science Academy and is a recipient of the Hari Om Ashram Prerit Senior Scientist Award. The Government of India honored him with Padma Shri, the fourth highest Indian civilian award for his contributions to the discipline of Physics, in 1982.
The impact of the solar wind onto the magnetosphere generates an electric field within the inner magnetosphere - the convection field-. Its general direction is from dawn to dusk. The co-rotating thermal plasma within the inner magnetosphere drifts orthogonal to that field and to the geomagnetic field Bo. The generation process is not yet completely understood. One possibility is viscous interaction between solar wind and the boundary layer of the magnetosphere (magnetopause). Another process may be magnetic reconnection. Finally, a hydromagnetic dynamo process in the polar regions of the inner magnetosphere may be possible. Direct measurements via satellites have given a fairly good picture of the structure of that field. A number of models of that field exists.
In the height region between about 85 and 200 km altitude on Earth, the ionospheric plasma is electrically conducting. Atmospheric tidal winds due to differential solar heating or due to gravitational lunar forcing move the ionospheric plasma against the geomagnetic field lines thus generating electric fields and currents just like a dynamo coil moving against magnetic field lines. That region is therefore called ionospheric dynamo region. The magnetic manifestation of these electric currents on the ground can be observed during magnetospheric quiet conditions. They are called Sq-variations and L-variations (L=lunar) of the geomagnetic field. Additional electric currents are generated by the varying magnetospheric electric convection field. These are the DP1-currents and the polar DP2-currents. Finally, a polar-ring current has been derived from the observations which depends on the polarity of the interplanetary magnetic field. These geomagnetic variations belong to the so-called external part of the geomagnetic field. Their amplitudes reach at most about 1% of the main internal geomagnetic field Bo.
Francisca Nneka Okeke is a Nigerian physicist. She is a Professor of Physics at the University of Nigeria, Nsukka and first female head of a department in the University.
The solar storms of August 1972 were a historically powerful series of solar storms with intense to extreme solar flare, solar particle event, and geomagnetic storm components in early August 1972, during solar cycle 20. The storm caused widespread electric‐ and communication‐grid disturbances through large portions of North America as well as satellite disruptions. On 4 August 1972 the storm caused the accidental detonation of numerous U.S. naval mines near Haiphong, North Vietnam. The coronal mass ejection (CME)'s transit time from the Sun to the Earth is the fastest ever recorded.
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