Main Phase Of Magnetic Storm Research Articles

A multievent study of broadband electrons observed by the DMSP satellites and their relation to red aurora observed at midlatitude stations

Broadband electrons during magnetic storms are characterized by an unusually intense flux of precipitating electrons in the broadband energy range from 30 eV to 30 keV near the equatorward edge of the auroral oval (47°–66° magnetic latitude). Broadband electrons were first reported by Shiokawa et al. [1996]. In this paper, we report a multievent study of broadband electrons, using particle data obtained by the Defense Meteorological Satellite Program (DMSP) satellites during 23 magnetic storms from January 1989 through May 1992. Twelve broadband electron events are identified. Most of them are observed in the night sector, but some are observed in the morning sector. Particle data for successive polar passes of the DMSP multisatellites are used to show that broadband electrons generally last for less than 30 min and that for some events, they precipitate over a wide range of local times simultaneously. On the basis of a quantitative calculation of optical emissions from electrons in the neutral atmosphere, we conclude that broadband electrons are a possible cause of red auroras observed at midlatitude ground stations. We suggest that broadband electrons are associated with certain substorms during the main phase of magnetic storms. This conjecture comes from observations of H component positive bays and Pi 2 pulsations observed at low‐latitude magnetic stations and from magnetic field variations observed at geosynchronous satellites. We conclude that the magnetospheric source of broadband electrons lies within the inner part of the plasma sheet. This conclusion is based on the facts that broadband electrons appear in latitudes where plasma sheet particles were observed before the event and that broadband electrons are observed poleward of the subauroral ion drifts, a position that corresponds to the inner edge of the injected particle layer during storms. High‐energy particle data obtained at geosynchronous satellites show that both strong magnetopause compressions in the dayside and decrease of the particle fluxes in the nightside occur in association with the broadband electron events. Possible mechanisms of broadband electron production are discussed.

Broadband electrons observed by the DMSP satellites during storm‐time substorms

Electrons that have intense broadband spectra are found to precipitate in the equatorward part of the auroral oval. The electrons are observed by the DMSP satellites in association with particular substorms during the main phase of magnetic storms. Electron fluxes are drastically enhanced after the substorm onset at all the energies measured (32 eV to 30 keV) at magnetic latitudes lower than 60° (L < 4). The energy flux of the electrons exceeds 1.0 × 1013 eV cm−2 s−1 sr−1. The intense electron precipitation lasts less than 1 h. These characteristics are significantly different from those seen in the central plasma sheet region. We suggest that an unknown energization mechanism of these electrons exists at the inner part of the plasma sheet at L < 4 during these particular storm‐time substorms.

Substorms, Storms, and the Near-Earth Tail.

More than 50, 000 measurements made with the AMPTE IRM satellite in the magnetotail at radial distances between 10 and 19 RE during 40 substorms have been used in a super-posed epoch analysis, separating storm-time substorms from those occurring during intervals of low Dstt levels. It is found that most of the tail signatures reported previously as typical for substorms are strongly influenced by magnetic storm activity. The magnetic field dipolarization and the Earthward convection in the near-Earth tail are much more pronounced for substorms which occur during the main phase of magnetic storms than for non-storm substorms. Moreover, only for storm-time substorms the lobe magnetic pressure decreases during the expansion phase. These findings indicate that not all substorms are alike and that the near-Earth neutral line scenario may apply only to the storm-time substorms.

Meso-scale structures of radiation belt/ring current detected by low-energy ions

We show examples of newly found fine structures in low-energy (< 1 keV) ion distribution inside the ring current/radiation belt region. They are, from equatorward to poleward, (1) wedge-like energylatitude dispersion of high intensity oxygen, (2) irregular field-aligned low-energy (<200 eV) ion flow, and (3) wavy structures of field-aligned ions. They are primarily observed in the morning sector. The “wedge-like” structure continuously exists for several hours whereas the wavy structure is found when the total background ion intensity is high, i.e., during the main phase of magnetic storms. However, the precise conditions for these phenomena to occur are still unknown.

Equatorial electromagnetic emission with discrete spectra near harmonics of oxygen gyrofrequency during magnetic storm

Equatorial electromagnetic ELF emissions at a frequency range around the oxygen ion gyrofrequency were observed in the low altitude plasmasphere (L = 1.5∼2.5) during magnetic storms with high activity of the ring current by triaxial search coil and single component electric field instruments on board the Akebono satellite. We find that a particular type of the ELF emissions occur with a multi‐band spectral structure around the local gyrofrequency (Fo+) of the oxygen ion (O+) during the main phase of magnetic storm. Our finding includes the following; The multi‐band emissions are localized in the morning sector and often consist of several spaced frequency bands closely related the local gyro‐harmonics of oxygen ion (O+) with a fundamental frequency of Fo+ or 2Fo+. The frequency of each band is dependent on the local magnetic field variation along the satellite orbit and the spacing between the bands is almost equal to Fo+ or 2Fo+. It is suggested that emissions are generated by terrestrial oxygen ion composing the ring current area during the main phase of magnetic storm.

Characteristics of Magnetic Variations Caused by Low-Latitude Aurorae Observed around 210.DEG. Magnetic Meridian.

Optical and magnetic observations at the Moshiri Observatory (L = 1.6) of the Solar-Terrestrial Environment Laboratory, Nagoya University, show that invisible low-latitude aurorae sometimes appear in concert with ≥50-nT H component positive magnetic excursions and large-amplitude Pi pulsations during the main phase of magnetic storms, for example, on February 9, 26, 27, and 29 and May 10, 1992, even with a minimum Dst index of only ∼-130 nT. Magnetograms from the L = 1.03-2.13 stations of the 210° magnetic meridian show that the positive excursions in ΔH begin almost simultaneously in the northern and southern hemispheres during the low-latitude aurorae. However, there is large northern-southern asymmetry in the D component magnetic variations during the low-latitude amoral events. The D component magnetic perturbations localized around L = 2 in the southern hemisphere are nearly time derivative of the H component; that is, a bipolar structure. The H and D component magnetic variations have apparent phase delays. The equivalent ionospheric current pattern moves from the midnight side toward the evening and morning sides during the premidnight and postmidnight sectors, respectively. The apparent longitudinal velocity of the equivalent ionospheric current pattern is about 7 km/s at around 40° magnetic latitude. Equivalent ionospheric current vectors deduced from the magnetic variations on the ground in the southern hemisphere show a vortex structure with clockwise rotation. These magnetic variations suggest that an ionospheric Hall current vortex, in which a field-aligned current of 1.4 x 10-8 A/m2 flows upward, must be formed locally near the low-latitude aurorae.

HF Doppler observations of vector plasma drifts in the evening F-region at the magnetic equator

Using HF Doppler radar in a spaced-receiver configuration, observations conducted on the vector plasma drifts in the equatorial F-region at Trivandrum during the afternoon-midnight period of March–April 1988, are presented. The three components of the F-region plasma drift at the magnetic equator are such that the North-South component corresponds to the meridional neutral air wind while the other two components represent the electrodynamic drift. The vertical component of the plasma drift is characterized by a prereversal upward enhancement which fits the interpretation based on the F-region dynamo operating at sunset. Equatorial spread-F, which is linked to the prereversal enhancement, appears to be inhibited during the main phase of magnetic storms. The East-West component of the plasma drift is westward during the afternoon and eastward during the night with mean peak velocities of 30 and 110 ms −1, respectively. The large night-time drift is believed to be due to a large reduction in the degree of electromagnetic coupling between the E- and F-regions during the night. The North-South component, representing the meridional neutral air wind, is found to be northward during the afternoon and southward during the night with respective mean peak velocities of about 50 and 70 m s −1. The neutral air wind shows a tendency to reverse from southward to northward at around midnight and is thought to be caused by the midnight bulge in the neutral air temperature. The observations which correspond to the lower F-region are found to be consistent with those reported earlier using other experimental techniques.

Is Substorm Occurrence a Necessary Condition for a Magnetic Storm?

It is shown that “having substorms” is not a necessary condition for a magnetic storm. The main phase of magnetic storms develops because of sustained, southward interplanetary magnetic field (IMF), not because of frequent occurrence of intense substorms.

Supply of energy to the magnetosphere during the main phase of magnetic storms

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A comparison of the 0.1–17 keV/e ion composition in the near equatorial magnetosphere between quiet and disturbed conditions

Data from the plasma composition experiment on ISEE 1 were used to investigate the relative number densities and energy characteristics of H+, He++, He+, and O+ in the near‐equatorial magnetosphere at L ≤ 12 during magnetically quiet and disturbed conditions. The ‘quiet’ data were selected on the basis of both Dst and Kp over ∼5 day periods preceeding the data accumulations, and the ‘active’ data were from periods of Dst ≲ −100γ. The ions included in this study had energies in the range of 0.1 ≤ E/Q ≲ 17 keV/e. The data naturally subdivide into two largely different sets in terms of a dipole L parameter. (1) At L ≲ 5 the ion density is dominated by three species, H+, O+, and He+, appearing in variable ratios. The He++ is a small component and is generally obscured by background counts caused by penetrating MeV electrons. The O+ is the most abundant species in well over half of the plasma samples, whereas the H+ is more abundant in the remaining samples. No sample with statistically significant counts shows He+ to be the most abundant ion, although the He+ exceeds the H+ in density in several cases. The O+/H+ and He+/H+ ratios increase toward the inner edge of the ring current population and are, on the average, highest during quiet conditions. Both of these features are qualitatively consistent with a decay process dominated by charge exchange, but a comparison with published charge exchange rates seems to imply that other processes are also involved. All three ion species have a similar mean energy (∼2–7 keV) within the energy window of the experiment, and all three have a low‐energy component (0.1 ≤ E/Q ≲ 1 keV/e) that usually dominates the number density at the inner edge, at about L ∼ 3–4 during quiet conditions and L ∼ 2–3 during disturbed conditions. (2) At L &gt; 5 all four species may be present simultaneously in highly variable ratios, but the H+ is usually the most abundant, particularly during quiet conditions. As a rule, the O+/H+ and He+/H+ ratios decrease with increasing L, whereas the He++/H+ ratio slowly increases. Typical values in the outer magnetosphere (L ∼ 10–12) are 1–3% for the He++/H+ and He+/H+ ratios during both quiet and disturbed conditions, whereas the O+/H+ ratio may range from below 1% to over 200%, depending on the magnetic activity level. The highest O+/H+ ratios are found here during the early main phase of magnetic storms, which is contrary to the conditions at L &lt; 5. The H+, O+, and He+ all have a similar mean energy in this region (∼4–6 keV), whereas the He++ is 2‐3 times more energetic.

Energetic neutral atom precipitation as a possible source of midlatitude F region winds

Midlatitude F region winds velocities observed during main phases of magnetic storms have been shown by Hernandez and Roble to be correlated with the rate of energy input into the ring current as measured by d (Dst)/dt.The attempt by Hernandez and Roble to relate the winds to the ring current decay mechanism of Cummings and Dessler fails because a correlation of wind velocity with positive values of d (Dst)/dt is necessary to support this mechanism rather than the observed correlation with negative values of d (Dst)/dt.Thus an alternative mechanism for deposition of ring current associated energy into the midlatitude thermosphere is needed. The first event discussed by Hernandez and Roble shows a wind divergence at midlatitudes which is taken as evidence of direct energy input there, and there are several suitable mechanisms, some or all of which would act in addition to high latitude sources (clearly present in the absence of changes in Dst).The mechanism of the precipitation of energetic neutral atoms arising from the charge exchange neutralization of ring current ions is suitable in all respects for a direct midlatitude energy input, except that the energy flux is several times too small, but there are reasons for the flux being greater than hitherto inferred.

Stormtime Pc 5 magnetic pulsations observed at synchronous orbit and their correlation with the partial ring current

Stormtime Pc 5 pulsations are a class of magnetic pulsations observed at synchronous orbit most frequently during the main phase of magnetic storms. The waves are linearly polarized in a meridian plane with nearly equal compressional and transverse components. The waves usually accompany a depression in local field magnitude associated with the onset of a magnetospheric substorm. In this paper, we use local time‐universal time maps of mid‐latitude magnetic perturbations to study the association of these waves with the development of the partial ring current. We find that these waves occur at the local time of maximum development of the partial ring current, at or shortly after the onset of a magnetospheric substorm. These results confirm our earlier speculation based on satellite data alone that the waves are associated with substorm enhancement of the partial ring current.

S³-A spacecraft and experiment description

The scientific objectives of the Explorer-45 mission are discussed. The primary objective is the study of the ring current responsible for the main phase of magnetic storms. Closely associated with this objective is the determination of the relationship between magnetic storms, substorms, and the acceleration of charged particles in the magnetosphere. Further objectives are the measurement of a wide range of proton, electron and alpha-particle energies, and studies of wave-particle interactions responsible for particle transport and loss in the inner magnetosphere. The orbital parameters, the spacecraft itself, and some of its unique features, such as the data handling system, which is programmable from the ground, are described.

Storms and the seasonal anomaly in the topside ionosphere

Data from the Alouette-I topside sounder have been used to study the height and latitudinal electron distributions during the seasonal anomaly and during ionosphere storms. It is shown that in the daytime topside ionosphere, disturbance effects having similar features to those of the seasonal anomaly occur mainly during the main phase of magnetic storms and sometimes during active periods of the recovery phase. The importance of these characteristics in understanding the nature of the two phenomena is discussed. An attempt is made to explain quantitatively the seasonal anomaly as caused by changes in atmospheric composition near the turbopause ( n(O) decrease and n(N 2) and n(O 2) increase from winter to summer). The disturbance effects in the daytime F2-region and in the topside ionosphere during the main phase of magnetic storms are also interpreted in terms of changes in atmospheric composition near turbopause levels.

Relativistic electron precipitation during magnetic storm main phase

Relativistic electron precipitation during magnetic storms, showing cyclotron resonances with electromagnetic ion cyclotron waves

F-region heating observed during the main phase of magnetic storms

The variation of the F-region electron temperature observed at Millstone Hill during geomagnetic storms that occurred in June, August, and September 1965 is discussed. These measurements, made by using the incoherent (Thomson) scatter technique, show the existence of nocturnal heating of the electrons. In the June storm there is also evidence of an anomalously large heat flux (∼1010 ev/cm²/sec) from the magnetosphere into the F region during the daytime. All periods of heating appear to occur during the main phase of the storm, as observed at Honolulu, and are usually accompanied by marked decreases in the intensity of the total field observed locally. On the basis of the present observations it is not possible to exclude precipitation of low-energy particles as the heating mechanism, but a more likely explanation seems to be that the heat is generated through the production or decay of the ring current and is conducted along field lines into the ionosphere. As such these observations provide support for the heat conduction theory developed by K. D. Cole (1955) to explain the stable red arc.

Tail of the magnetosphere

The Chapman and Ferraro [e.g., Beard, 1960] model of the magnetosphere is entirely dissipationless and, as a consequence, contains no tangential stress at the magnetosphere boundary. It has been suggested that either viscous interaction [Axford and Hines, 1961; Piddington, 1963] or reconnection of field lines [Dungey, 1961; Levy, Petschek, and Siscoe, 1963] can give rise to tangential stresses which produce internal convection within the magnetosphere. A number of auroral and ionospheric observations appear to be consistent with such a convection pattern. It was also suggested [Piddington, 1960] that tangential stresses might stretch the magnetosphere tail and thus account for the main phase of magnetic storms.

The main phase of magnetic storms and the ring current

The ring current, a westward electric current encircling the earth which is responsible for the main phase decrease of magnetic storms, is discussed. Present knowledge of the ring current is summarized, and several important problems that must be explored further are discussed. The motion of charged particles in a dipole field the electric current resulting from this motion, and the magnetic fields produced by the ring current are considered. The magnetic moment of the ring current, its deformation effects on the outer radiation belt, and its growth time are also discussed. (C.E.S.)

The development of the main phase of magnetic storms

The development of the main phase and of auroral activities during magnetic storms is examined for a number of storms. It varies much more than a mere difference of pressure of the solar streams would seem to produce. Some intrinsic differences between soiar streams are suggested. (auth)

Cosmic ray evidence for a ring current

Many observations show that low rigidity protons from solar flares are permitted entry at Minneapolis only during the main phase of magnetic storms. The measured energy is much below the normal Stormer cutoff at that time. This paper develops the idea that a ring current responsible for the main storm field reduces the Stormer cosmic-ray cutoffs. The model of the ring chosen is actually an azimuthal current on the surface of a sphere with current intensity proportional to sin θ where θ = colatitude. It is shown that this mathematical form permits great simplicity in the analysis and leads to essentially the same result as a diffuse ring corresponding to the actual trapped radiation. The magnetic moment of the ring required to produce the cutoff change can be provided by reasonable intensities of very low energy trapped radiation. It is shown that the cosmic-ray data permit one to evaluate both the moment (M′) and radius (R) of the ring, whereas the surface magnetic measurements determine only the quantity M′/R3. Since observation shows that the cosmic-ray cutoffs return to normal during the main phase, it must be assumed that the ring shrinks inward so that the surface field is maintained negative. Cosmic-ray evidence concerning the presence of a permanent ring current is discussed. Such a ring is measured directly by satellites during quiet times.