Low-altitude Polar-orbiting Satellite Research Articles

A dataset of field-aligned Poynting flux in the dayside polar cap of the northern hemisphere during 2014-2016

The transport of electromagnetic energy in the polar upper atmosphere and its spatial and temporal distribution are current hot issues and important topics for studying the polar upper atmospheric environment and space weather forecasting models. Researchers usually adopted the observations from low-altitude polar-orbiting satellites to estimate the magnitude and direction of electromagnetic energy. The continuous accumulation of electromagnetic energy data will be beneficial to the study on the transmission and variation characteristics of electromagnetic energy in the polar ionosphere in China. In this paper, we estimated the distribution of field-aligned Poynting flux, horizontal convection electric field and magnetic field perturbation in the dayside polar cap based on the plasma velocity and magnetic field data of DMSP F17 satellite in the northern hemisphere during 2014-2016. The initial DMSP F17 satellite data included in this dataset are of high quality. The estimation process is rigorous and reasonable, and the data records are complete. The dataset can provide important data support for the study on energy transport and dissipation in the ionosphere/thermosphere in the polar region.

Gamma rays in <i>L-B</i> coordinates at CORONAS-I altitude

Abstract. We present here observations of gamma rays in the energy range between 3.0 and 8.3 MeV gathered by the SONG instrument aboard low-altitude polar-orbiting satellite CORONAS-I throughout the period March-June 1994. We concentrate on the emissions related to the trapped particles and organize CORONAS-I measurements in the magnetic L–B coordinate system. The spatial distribution of the average gamma-ray counts reveals that the most intense fluxes were observed under the inner radiation belt, at L<2, and that they are exclusively confined into the region of stably trapped particles, where daughter gamma rays could result from the interactions within the spacecraft and instrumental matter. In the outer radiation zone (L~4), the enhanced gamma radiation, also detected outside the stably trapping region, shows pronounced longitudinal variations. The observed eastward increase in the gamma-ray count rate suggests quasi-traped energetic (megavolt) electrons as a source of the gamma rays both in the upper atmosphere and in the satellite matter, most likely, through the bremsstrahlung process in the studied energy domain. Keywords. Magnetospheric physics (Energetic particles, precipitating; Energetic particles, trapped; Magnetosphereionosphere interactions)

Review of electron fluxes within the local drift loss cone: Measurements on CORONAS-I

Measurements of relativistic electrons (500 keV to 12 MeV) within the drift loss cone during geomagnetic storms in the spring of 1994, obtained with MKL instrument aboard the low-altitude (∼500 km) polar-orbiting satellite, CORONAS-I, are reviewed. CORONAS-I satellite observed two more pronounced electron enhancements during the period studied (March–April 1994). Time profile of Dst index indicates that these events occurred after magnetic storms on March 6–8, 1994 (Dst = −109 nT) and April 2–4, 1994 (Dst = −111 nT). Both time and L shell variations of the precipitating electron fluxes over the wide L shell range of 1–9 are presented and discussed. On the average, these fluxes had a maximum value at L = 3.8–4.6 observed during the storm recovery phases.

Response of auroral regions to directional changes of the interplanetary magnetic field: Case study

On October 6, 1979, the low-altitude polar-orbiting satellites DMSP-F2 and-F4 crossed the auroral electron precipitation region in the opposite hemispheres at nearly the same universal time (UT) and in the same magnetic local-time (MLT) sector near midnight. Three pairs of such nearly simultaneous conjugate crossings took place during a period of enhanced magnetic activity and strongly turning northward or southward of the interplanetary magnetic field (IMF). These conjugate observations allowed the study, with time resolution better than six minutes, of the variation, in response to directional changes of the interplanetary magnetic field, of the latitudinal position and width of the auroral regions; these are believed to map the central plasma sheet (CPS) and boundary plasma sheet (BPS). During the equatorward expansion of the whole auroral electron precipitation region, its latitudinal width is observed to decrease markedly when the IMF turns from a northern to a southern direction. In particular, a different response of the equatorward boundary of the auroral oval with respect to the poleward boundary results from the observations, showing that the speed of the equatorward expansion of the equatorward boundary, measured at a temporal resolution of less than 6 minutes, is lower than the speed of the poleward boundary. The BPS/CPS boundary moves coherently with the southward turning of the IMF, with intermediate speed. It follows that the latitudinal width of the poleward part of the auroral region, assumed to map the boundary plasma sheet, decreases more dramatically than the width of the equatorward part of the region mapping the central plasma sheet. These findings could be explained in terms of changes of the total open magnetic flux. Actually, the equatorward shift of the poleward boundary of the auroral oval and the subsequent dramatic thining of the BPS region seem to be the consequence of a larger number of geomagnetic flux line interconnected with the IMF during a southward IMF condition.

Low‐altitude signatures of the cusp and flux transfer events

The usual interpretation of a flux transfer event (FTE) at the magnetopause, in terms of time‐dependent and possibly patchy reconnection, demands that it generate an ionospheric signature. Recent ground‐based observations have revealed that auroral transients in the cusp/cleft region have all the characteristics required of FTE effects. However, signatures in the major available dataset, namely that from low‐altitude polar‐orbiting satellites, have not yet been identified. In this paper, we consider a cusp pass of the DE‐2 spacecraft during strongly southward IMF. The particle detectors show magnetosheath ion injection signatures. However, the satellite motion and convection are opposed, and we discuss how the observed falling energy dispersion of the precipitating ions can have arisen from a static, moving or growing source. The spatial scale of the source is typical of an FTE. A simple model of the ionospheric signature of an FTE reproduces the observed electric and magnetic field perturbations. Precipitating electrons of peak energy ∼100eV are found to lie on the predicted boundary of the newly‐opened tube, very similar to those found on the edges of FTEs at the magnetopause. The injected ions are within this boundary and their dispersion is consistent with its growth as reconnection proceeds. The reconnection potential and the potential of the induced ionospheric motion are found to be the same (≃25kV). The scanning imager on DE‐1 shows a localised transient auroral feature around DE‐2 at this time, similar to the recent optical/radar observations of FTEs.

Recent ionospheric observations relating to solar-wind-magnetosphere coupling

Simultaneous observations in the high-latitude ionosphere and in the near-Earth interplanetary medium have revealed the control exerted by the interplanetary magnetic field and the solar wind flow on field-perpendicular convection of plasma in both the ionosphere and the magnetosphere. Previous studies, using statistical surveys of data from both low-altitude polar-orbiting satellites and ground-based radars and magnetometers, have established that magnetic reconnection at the dayside magnetopause is the dominant driving mechanism for convection. More recently, ground-based data and global auroral images of higher temporal resolution have been obtained and used to study the response of the ionospheric flows to changes in the interplanetary medium. These observations show that ionospheric convection responds rapidly (within a few minutes) to both increases and decreases in the reconnection rate over a range of spatial scales, as well as revealing transient enhancements which are also thought to be related to magnetopause phenomena. Such results emphasize the potential of ground-based radars and other remotesensing instruments for studies of the Earth’s interaction with the interplanetary medium.

Precipitation of electrons after geomagnetic substorms

Precipitating electrons Ee>10 keV and Ee>30 keV and protons Ep=150 to 300 keV are analyzed using data obtained from the low-altitude polar-orbiting satellite Interkozmos-17 after the magnetic storm of December 2, 1977. On December 3, an isolated substrom occurred, and strong electron precipitation was observed in the midnigt sector, while the sequence of substorms of December 5 expanded its longitudinal extent from 22 MLT through the morning to 12 MLT. The different character of the precipitation, in the two cases mentioned, is discussed in relation to the changes of conditions for the generation of cyclotron instability as well as to the effect of particle injection. Proton precipitation occurred only in the night sector, most probably just inside the plasmapause.

The effect of a geomagnetic storm on energetic trapped protons at low altitudes

The irreversible changes of the intensity of trapped protons with energy above 1 MeV in the Earth's magnetosphere near the outer boundary of trapping are observed after moderate geomagnetic storms on the low-altitude polar-orbiting satellite Intercosmos-17. These changes are interpreted in terms of nonadiabatical effects of proton motion in the disturbed geomagnetic field (assuming D st variation) which affects the conditions for stable trapping of protons during the storm. The decrease of proton intensity is due to an adiabatic decrease of energy, an increase of mirror-point altitude and nonadiabatic scattering and losses. The interaction of two types of particle motion—gyrorotation and the ‘bounce’ motion, which leads to the instability of motion, is assumed. The importance of nonadiabatical losses of trapped protons with low equatorial pitch angles for changes near the proton boundary is pointed out.

Evidence for very weak pitch angle diffusion of outer zone electrons

Integral fluxes of locally trapped and loss cone electrons >30 keV were measured with the low-altitude polar-orbiting satellite Ogo 6. In the outer zone, weak pitch angle diffusion region, the observed ratios of precipitated to locally trapped electron intensities were generally a few percent. However, the ratios of backscattered to precipitated electrons were often greater than one in situations where they should not exceed unity. We show that both loss cone detectors were usually counting a significant flux of locally trapped electrons scattered off the walls of their respective collimators. Under conditions leading to a minimum of wall-scattered electrons in the precipitation loss cone detector, the upper limit of the ratio of precipitated to locally trapped electrons can be set at 5 x 10/sup -4/ to 10/sup -3/. These values are consistent with theoretical calculations for weak (<30 m..gamma..) whistler mode magnetic field amplitudes of the scattering waves. We suggest that previously reported loss cone/trapped flux ratios of a few percent (and the diffusion coefficients derived therefrom) are either incorrect or do not represent minimum diffusion conditions.

The effect of strong pitch angle scattering on the location of the outer-zone electron boundary as observed by low-altitude satellites

By using simultaneous observations of low-altitude energetic electron fluxes from the magnetic electron spectrometer on OV3-3 and equatorial electron fluxes observed by ATS 1 a previously identified strong pitch angle scattering mechanism is shown to be a low-altitude phenomenon only. The relationship between this scattering process and the apparent location of the outer-zone energetic electron boundary as observed by low-altitude polar-orbiting satellites is demonstrated. The process explains observations of rapid electron boundary motions during geomagnetically quiet times. In addition, by assuming that the process also explains the energetic electron spike sometimes seen at the apparent outer-zone cutoff the magnitude of the spike itself is used to determine the high-altitude limit of the rapid-scattering region.

The local time variation of ELF emissions during periods of substorm activity

A statistical study is reported concerning the occurrence probability of ELF emissions observed on the low-altitude polar-orbiting satellite OGO 6 during periods of substorm activity. Over 160 individual substorm periods have been selected and analyzed during the period from June 1969 to October 1970. The statistical results are discussed, taking into account ELF response to substorm activity and emissions during the recovery phase of magnetic storms.

Narrow spikes in the selective precipitation of relativistic electrons at mid-latitudes

Relativistic electron precipitation spikes have been observed in the quasi-trapped population at mid-latitudes (L ≈ 3.5–4.5). The spike electrons are not in the local loss cone but precipitate into the atmosphere upon drift in longitude. In these spikes, often as narrow as 0.3° in invariant latitude, the fluxes of electrons ≳4 MeV may undergo pronounced enhancements, whereas the intensities of electrons in the few hundred keV range may not be significantly affected. This energy selectivity in the formation of the spikes is perhaps more suggestive of wave-particle interactions, such as the Thorne and Kennel (1971) mechanism for the scattering of relativistic electrons by Doppler-shifted ion cyclotron waves, rather than of changes in the magnetic field topology. The data were taken from the low-altitude (∼750 km) polar-orbiting satellite (1972–076B) with an array of several electron detectors having widely differing geometric factors and energy ranges of response. Energy spectra and pitch angle distributions were measured over the energy range 160 keV to 2.4 MeV with a combination of three spectrometers. Fluxes of energetic (>4 MeV) electrons were measured with a high sensitivity in each of two large (∼3.4 kg) plastic scintillator anticoincidence counters surrounding a germanium spectrometer.

A coordinated study of energetic electron precipitation andDregion electron concentrations over Ottawa during disturbed conditions

This article presents the results of a study of the energetic electron precipitation as well as important parameters of the D region over Ottawa (45°N, 76°W) during lightly to moderately disturbed conditions, using data for the period following the December 17, 1971, magnetic storm as well as data obtained on other selected days exhibiting anomalous D region absorption. Following the December 17, 1971, storm, significant fluxes of precipitating electrons of > 130 keV were observed near Ottawa, even during the poststorm period on December 20, when the geomagnetic activity had subsided (∑ Kp = 6). The excess ionization detected at D region altitudes above Ottawa can be explained as being due to ionization from a prolonged electron drizzle from the outer radiation belt. For the first time it has been proven that precipitating electrons from the radiation belt were the main cause of D region poststorm conditions of middle latitudes. The ground-based radio probing measurements of the free electron concentration have been coordinated with simultaneous satellite observations of the quasi-trapped and precipitating electron environment above the site. From the ground-based measurements, by using the partial reflection technique at two frequencies (2.66 and 6.275 MHz), electron concentration profiles for altitudes between ≈ 60 and 90 km have been derived at times when the low-altitude polar-orbiting satellite 1971-089A passed near Ottawa. On board this satellite, electron differential fluxes in 260 channels and at two pitch angles were measured at energies between ≈1 and ≈2800 keV. The ion pair production rate height profiles due to precipitating electrons were computed for D region heights by using nonisotropic energy-dependent pitch angle distributions. Effective electron loss rates were deduced for heights between 63 and 91 km. These results indicate a significant variability in loss rates in the altitude range 75–85 km.

Solar protons on closed magnetospheric field lines after an interplanetary flux decrease

Particle measurements from the low altitude polar-orbiting satellite GRS-A/Azur and from Explorer 41 in the magnetosheath during a time period after the sudden commencement at 14:30 UT on 8 March 1970, have been used in order to study the access mode of solar particles into the closed field line region of the magnetosphere. A particle decrease in the magnetosheath and over the central polar cap but not in the stable trapping region indicates that solar particles are temporarily trapped and can complete several drifts around the Earth. A single loss cone distribution ∼2° inside of the stable trapping region cannot be explained by strong pitch angle scattering but is probably due to non-adiabatic particle motion.

Weak pitch angle scattering of energetic protons in the magnetosphere

During the solar particle event of April 12–14, 1973, a flux depression of solar protons in the dawn side pseudotrapping region relative to the flux in the dusk side pseudotrapping region was frequently observed with the low-altitude polar-orbiting satellite Esro 4. This flux decrease is interpreted as being due to weak pitch angle scattering of the solar protons during their drift from dusk to dawn. Treating the pitch angle diffusion as a simple initial value problem, we derive quantitative values of the equatorial bounce-averaged pitch angle diffusion coefficients at small pitch angles for different energies. These values represent lower limits for magnetospheric conditions at the time of the Esro 4 observations. Through the assumption that the pitch angle diffusion is a gyroresonant interaction process of the particles with either Alfven or ion cyclotron waves, the necessary power of the waves at the resonant interaction frequency is calculated.

On the angular distributions of electrons in ‘inverted v’ substructures

Data from the energetic particle detectors on board the low-altitude polar-orbiting satellite Isis 2 are presented to describe several features of the low-energy (0.15–9.7 keV) electrons in ‘inverted V’ substructures. The measurements show that, frequently, the fluxes of electrons in the inverted V events peak at ∼90° pitch angle. In addition to this principal peak the electron fluxes also have a tendency to once again increase at smaller pitch angles (usually ≲ 30°). This tendency becomes more significant at lower electron energies. On the basis of the measurements it is suggested that acceleration processes other than parallel electric fields are mainly responsible for the formation of at least some of the observed inverted V substructures and that such inverted V events probably occur on closed field lines in the magnetosphere.

Solar proton entry to the magnetosphere on 18 November 1968 and 25 February 1969—I. Interpretation of satellite data using trajectory computations in a model magnetosphere

Latitudinal distributions of high-energy (∼100 MeV) solar protons measured over the Earth's polar caps by low-altitude polar-orbiting satellites are described. The data, which refer to the initial phases of the 18 November 1968 and 25 February 1969 solar events, are related to interplanetary particle and magnetic-field measurements by means of trajectory computations using a model magnetosphere. A feature of this model magnetosphere, which is based on the Hones-Taylor model, is its realistic representation of the geomagnetic tail under relatively quiet magnetic conditions. It is shown that the proton distributions over the polar caps are consistent with the field-aligned particle anisotropies measured outside the magnetosphere in the initial phases of the two events. Lower-latitude maxima in the proton count rates are associated with particle entry through the dawn side of the magnetopause, and higher-latitude minima are associated with particle entry through the dusk side. Particle entry also occurs through the magnetotail and this mode of access produces enhancements of the proton count rates in the central polar caps. These results imply that solar protons had direct access to the magnetosphere during the two events.

Variations of the trapped particle population and of cutoff and pitch angle distribution of simultaneously observed magnetospheric solar protons during substorm activity

The behavior of solar protons (1.5–2.7 MeV) and trapped electrons (>1.5 MeV) observed by a low-altitude polar-orbiting satellite (Azur) during the occurrence of an isolated substorm (March 29–30, 1970) is examined. Onset of the substorm is determined and its development is discussed by using energetic and high-energy particle data from the ATS 5 satellite. The comparison of equatorial and low-altitude electron data shows that the behavior of the trapped electron population is consistent with the adiabatic model given by Lezniak and Winckler (1970). Observation of an isotropic pitch angle distribution of the solar protons leads to the conclusion that there is evidence for strong pitch angle scattering of these particles in the pseudotrapping region. We also observed during the substorm a large change in the slope of the solar particle flux profile at cutoff that occurred simultaneously with a variation of the trapping boundary. It is suggested that this effect might explain the often observed ‘deep entry’ of solar protons into closed drift shells well below the nominal cutoff.

Observed relationships between electric fields and auroral particle precipitation

Simultaneous electric field and plasma observations with the low-altitude polar-orbiting satellite Injun 5 have provided a comprehensive survey of convection electric fields and their association with magnetospheric plasma phenomena. The most prominent features of the convection electric fields are reversals located at high magnetic latitudes, with generally antisunward convection poleward and sunward convection equatorward of the electric field reversal location. The electric field reversal is interpreted as the boundary between open and closed magnetic field lines. During local day the electric field reversal is observed to coincide with the equatorward boundary of the polar cusp. The plasma flow in the dayside polar cusp region is dominantly E-W, away from the stagnation point, the convection velocities typically being about 1 km/sec. At local evening, 'inverted V' electron precipitation bands are observed near or at the position of the electric field reversal. In the local late-evening sector the electric field reversal becomes less distinct, and often no single well-defined electric field reversal can be identified. In all cases the inverted V electron precipitation events are closely associated with large, typically greater than 30 mV/m, irregular electric field fluctuations with time scales of a few seconds or less.

Polar-cap structures of solar protons observed during the passage of interplanetary discontinuities

Particle measurements from the low-altitude polar-orbiting satellite GRS-A/Azur and from the Imp 5 satellite inside the magnetosphere near the magnetopause and particle and magnetic-field measurements in interplanetary space from the Heos A1 satellite are presented for the solar-proton event of March 1970. Comparison of interplanetary magnetic-field and particle data indicate that a flux increase lasting 20 min is associated with an interplanetary magnetic filament. A following sharp flux increase is attributed to an interplanetary magnetic discontinuity. As the discontinuity with different proton fluxes on both sides is swept over the earth, the particles immediately enter the magnetosphere near the neutral sheet, and the higher flux is observed without time delay in the quasi-trapping region over the polar cap. The central polar cap is still dominated by particles entering via the distant magnetospheric tail. The discontinuity is convected along the tail and ultimately reaches the region where field lines originating in the central polar cap are connected to or intermingled with the interplanetary field. Filling up of the polar cap in less than 2 hours to the higher flux level results in a maximum ‘tail length’ of about 550 RE.