Small Magnetic Storm Research Articles

Extreme Positive Ionosphere Storm Triggered by a Minor Magnetic Storm in Deep Solar Minimum Revealed by FORMOSAT‐7/COSMIC‐2 and GNSS Observations

AbstractThis study examines an unexpected and extreme positive ionospheric response to a minor magnetic storm on August 5, 2019 by using global ionosphere specification (GIS) 3D electron density profiles obtained by assimilating radio occultation total electron content (TEC) measurements of the recently launched FORMOSAT‐7/COSMIC‐2 satellites, and ground‐based global navigation satellite system (GNSS) TEC. The results reveal ∼300% enhancement of equatorial ionization anomaly (EIA) crests, appearing over 200–300 km altitudes, and a much intense localized density enhancement over the European sector. These are the most intense ionospheric response that has ever been detected for a small magnetic storm with Dst ∼ −53 nT (SYM‐H ∼ −64 nT). The enhancements are validated by using global ionosphere map (GIM) TEC and ground‐based GNSS TEC. The GIS vertical electron density structures during the storm are examined to understand the physical processes giving rise to such an intense ionosphere response during deep solar minimum conditions when the background electron density is very low. Altitude variations and poleward shifts of the locations of the EIA crests indicate that prompt penetration electric fields (PPEF) play an important role in producing the observed positive storm responses, with the storm‐induced equatorward circulation supporting the plasma accumulation against recombination losses. Additional physical mechanisms are required to fully explain the unexpected electron density enhancements for this minor storm event.

Statistical Properties of Molecular Ions in the Ring Current Observed by the Arase (ERG) Satellite

AbstractMolecular ions in the magnetosphere can be a tracer of fast ion outflows from the deep ionosphere. Statistical properties of molecular ions (O2+/NO+/N2+) in the ring current are investigated based on ion composition measurements (<180 keV/q) by medium‐energy particle experiments‐electron analyzer and low‐energy particle experiments‐ion mass analyzer instruments on board the Arase (Exploration of energization and Radiation in Geospace, ERG) satellite. The investigated period from late March to December 2017 includes 11 geomagnetic storms with the minimum Dst index less than −40 nT. The molecular ions are observed in the region of L = 2.5–6.6 and clearly identified at energies above ~12 keV during most magnetic storms. During quiet times, molecular ions are not observed. The average energy density ratio of the molecular ions to O+ is ~3%. The ratio tends to increase with the size of magnetic storms. Existence of molecular ions even during small magnetic storms suggests that the fast ion outflow from the deep ionosphere occurs frequently during geomagnetically active periods.

Variations of the relativistic electron flux after a magnetospheric compression event

On January 21, 2015, a sharp increase of the solar wind dynamic pressure impacted the magnetosphere. The magnetopause moved inward to the region L< 8 without causing a geomagnetic storm. The flux of the relativistic electrons in the outer radiation belt decreased by half during this event based on the observations of the particle radiation monitor (PRM) of the fourth of the China-Brazil Earth Resource Satellites (CBERS-4). The flux remained low for approximately 11 d; it did not recover after a small magnetic storm on January 26 but after a small magnetic storm on February 2. The loss and recovery of the relativistic electrons during this event are investigated using the PRM data, medium- and high-energy electron observations of NOAA-15 and the Van Allen Probes, medium-energy electron observations of GOES-13, and wave observations of the Van Allen Probes. This study shows that the loss of energetic electrons in this event is related to magnetospheric compression. The chorus waves accelerate the medium-energy electrons, which causes the recovery of relativistic electrons. The Van Allen Probes detected strong chorus waves in the region L = 3–6 from January 21 to February 2. However, the flux of medium-energy electrons was low in the region. This implies that the long-lasting lack of recovery of the relativistic electrons after this event is due to the lack of the medium-energy “seed” electrons. The medium-energy electrons in the outer radiation belt may be a clue to predict the recovery of relativistic electrons.

Strange VLF bursts in northern Scandinavia: case study of the afternoon &amp;quot;mushroom-like&amp;quot; hiss on 8 December 2013

Abstract. We investigate a non-typical very low frequency (VLF) 1–4 kHz hiss representing a sequence of separated noise bursts with a strange "mushroom-like" shape in the frequency–time domain, each one lasting several minutes. These strange afternoon VLF emissions were recorded at Kannuslehto (KAN, &amp;amp;varphi; = 67.74° N, λ = 26.27° E; L ∼ 5.5) in northern Finland during the late recovery phase of the small magnetic storm on 8 December 2013. The left-hand (LH) polarized 2–3 kHz "mushroom caps" were clearly separated from the right-hand (RH) polarized "mushroom stems" at the frequency of about 1.8–1.9 kHz, which could match the lower ionosphere waveguide cutoff (the first transverse resonance of the Earth–ionosphere cavity). We hypothesize that this VLF burst sequence could be a result of the modulation of the VLF hiss electron–cyclotron instability from the strong Pc5 geomagnetic pulsations observed simultaneously at ground-based stations as well as in the inner magnetosphere by the Time History of Events and Macroscale Interactions during Substorms mission probe (THEMIS-E; ThE). This assumption is confirmed by a similar modulation of the intensity of the energetic (1–10 keV) electrons simultaneously observed by the same ThE spacecraft. In addition, the data of the European Incoherent Scatter Scientific Association (EISCAT) radar at Tromsø show a similar quasi-periodicity in the ratio of the Hall-to-Pedersen conductance, which may be used as a proxy for the energetic particle precipitation enhancement. Our findings suggest that this strange mushroom-like shape of the considered VLF hiss could be a combined mutual effect of the magnetospheric ULF–VLF (ultra low frequency–very low frequency) wave interaction and the ionosphere waveguide propagation.

First results of the high-resolution multibeam ULF wave experiment at the Ekaterinburg SuperDARN radar: Ionospheric signatures of coupled poloidal Alfvén and drift-compressional modes

A continuous experiment was carried out at the Ekaterinburg (EKB) stereoradar of the Russian segment of SuperDARN in order to examine the spatio-temporal characteristics of radar-detected magnetospheric ULF waves. The study of magnetospheric oscillations is based on analysis of scattering from field-aligned F-layer irregularities. Their E×B drift Doppler velocity at F-layer heights is associated with the background electric field in the ionosphere. During the experiment one of the radar channels operates in 0–2 beam scanning, with an integration time of 6s, which corresponds to the total 18-s time resolution at each beam. This allows detecting magnetospheric ULF waves with periods of 40s and up. Beam 0 is along the 132 magnetic meridian, so the registered velocity oscillations correspond to the wave electric field azimuthal component. Operation of the radar in this mode was started in December 2013.The first ULF wave events observed in the experiment and presented here occurred on 14 December 2013 and 2 January 2014 in the nightside magnetosphere during two geomagnetic disturbances classified as small magnetic storms and associated with high speed streams from coronal holes. Both the ULF events occurred after substorm-like auroral disturbances. The ULF waves observed during these events are classified as Pc5 geomagnetic pulsations. Two oscillation branches were observed, the higher and the lower frequency ones. As the azimuthal wave numbers m increase, the branches converge and merge into a single oscillation branch at some critical azimuthal wave number value m⋆. This ω(m) dependence is characteristic of the coupled Alfvén and drift-compressional waves which according to theory merge if the azimuthal wave number exceeds some critical value. This merged single oscillation branch represents an unstable drift ballooning coupling mode. Thus, the following interpretation of the observed events can be suggested; at m<m⋆ the higher and lower frequency branches can be identified with the Alfvén and drift-compressional modes, respectively, and at m>m⋆ the single branch can be identified with the drift ballooning coupling mode.

High-latitude geomagnetic disturbances during the initial phase of a recurrent magnetic storm (from February 27 to March 2, 2008)

A complex of geophysical phenomena (geomagnetic pulsations in different frequency ranges, VLF emissions, riometer absorption, and auroras) during the initial phase of a small recurrent magnetic storm that occurred on February 27–March 2, 2008, at a solar activity minimum has been analyzed. The difference between this storm and other typical magnetic storms consisted in that its initial phase developed under a prolonged period of negative IMF Bz values, and the most intense wave-like disturbances during the storm initial phase were observed in the dusk and nighttime magnetospheric sectors rather than in the daytime sector as is observed in the majority of cases. The passage of a dense transient (with Np reaching 30 cm−3) in the solar wind under the southward IMF in the sheath region of the high-speed solar wind stream responsible for the discussed storm caused a great (the AE index is ∼1250 nT) magnetospheric substorm. The appearance of VLF chorus, accompanied by riometer absorption bursts and Pc5 pulsations, in a very long longitudinal interval of auroral latitudes (L ∼ 5) from premidnight to dawn MLT hours has been detected. It has been concluded that a sharp increase in the solar wind dynamic pressure under prolonged negative values of IMF Bz resulted in the global (in longitude) development of electron cyclotron instability in the Earth’s magnetosphere.

Substorms during different storm phases

Abstract. After the deep solar minimum at the end of the solar cycle 23, a small magnetic storm occurred on 20–26 January 2010. The Dst (disturbance storm time) index reached the minimum of −38 nT on 20 January and the prolonged recovery that followed the main phase that lasted for about 6 days. In this study, we concentrate on three substorms that took place (1) just prior to the storm, (2) during the main phase of the storm, and (3) at the end of the recovery of the storm. We analyse the solar wind conditions from the solar wind monitoring spacecraft, the duration and intensity of the substorm events as well as the behaviour of the electrojet currents from the ground magnetometer measurements. We compare the precipitation characteristics of the three substorms. The results show that the F-region electron density enhancements and dominant green and red auroral emission of the substorm activity during the storm recovery resembles average isolated substorm precipitation. However, the energy dissipated, even at the very end of a prolonged storm recovery, is very large compared to the typical energy content of isolated substorms. In the case studied here, the dissipation of the excess energy is observed over a 3-h long period of several consecutive substorm intensifications. Our findings suggest that the substorm energy dissipation varies between the storm phases.

MDS-1 Observations of Highly Energetic Electron Environment in the Inner Magnetosphere

MDS-1 satellite observations demonstrated that highly energetic electrons in the outer radiation belt disappear during the main phase of the magnetic storm, and rebuilding of the highly energetic electrons is made during the recovery phase of the magnetic storm. A distribution of the new peak of highly energetic electron flux with respect to the distance from the Earth is inversely proportional to the magnitude of the magnetic storm. In case of the large storm, the outer electron belt is pushed toward the Earth, filling so-called slot region. It is of interest to identify that seed electrons are necessary around inner edge of the outer radiation zone to fill slot region. It was found that the location of the intense low frequency plasma waves, which appear during the storm recovery phase, coincides with the location of the peak intensity of the highly energetic electrons in the outer radiation belt. This coincidence strongly suggests that an internal acceleration process takes place which leads to a large increase in the intensity of highly energetic electrons in the outer radiation belt during the storm recovery phase. A seasonal variation of the outer radiation belt is identified. The increase in the intensity of the highly energetic electrons is large both in spring and autumn seasons. The magnetic activity is also large both in spring and autumn seasons. Correlation of the increase in the intensity of highly energetic electrons with the magnetic activity is identified. A solar cycle variation of the outer radiation belt is also identified. The location of the outer radiation belt was found closer to the Earth during the solar maximum periods and far from the Earth during the solar minimum periods. This variation is due to the evidence that large magnetic storms occur largely during the solar maximum periods, while small magnetic storms take place largely during the solar minimum periods, resulting in a long-term solar cycle variation with respect to the distance from the Earth.

Response of the thermosphere to Joule heating and particle precipitation

We have used output from the Weimer Joule heating model (2005) and the Air Force High Accuracy Satellite Drag Model (HASDM) to study the response of the thermosphere to Joule heating. Our study period of 15 January to 29 June 2001 contains a number of large and small magnetic storms during which thermospheric heating events occurred. We find that a new Joule heating model (Weimer, 2005), combined with the energy input provided by precipitating particles (NOAA/TIROS hemispheric power index), can supply more than enough energy to account for the change in total thermospheric internal and gravitational potential energy during magnetic storms. In the smaller storm heating events the energy input is about equally divided, with Joule heating only slightly dominant over particle precipitation. In the larger events, Joule heating clearly dominates. We find that the thermosphere responds globally in just 3–6 hours to an increase in energy input.

Dynamics of energetic ions and electrons between L = 2.5 and L = 7 during magnetic storms

The variation of energetic electron and ion fluxes with L values between 2.5 and 7 as measured by two magnetic spectrometers (medium electrons A and B, MEA and MEB) on CRRES will be presented for one intense and two weak magnetic storms, paying particular attention to the rate of accompanying substorms. Electrons in the energy range 45–602 keV and ions in the range 77–703 keV will be examined. The data are presented in cuts through L versus time plots which are ideal to demonstrate the global aspect and the long‐term variations of a magnetic storm in the outer radiation belt. The time resolution is half an orbital period of CRRES (5.5 hours). Substantial decreases and enhancements of electron and ion intensity are seen at the beginning and recovery of intense magnetic storms (Dst &lt; −100 nT). Electrons and ions are shifted to lower L values during these storms and the slot region narrows for ions. The slot region can disappear completely for electrons during the recovery of the storm. For small magnetic storms (Dst about −30 nT), which seem to be caused by a series of substorms, the ring current does not move to lower L values and stays at L values of about 5.5 but weakens, even though Dst decreases. This is attributed to the strong substorm current systems. Knowledge of the past history of the radiation belts is required to separate “true” storm time depressions of Dst from substorm‐related effects.

Substorm-injected protons and electrons and the injection boundary model

Observations of substorm-associated enhancements of proton and electron fluxes were made by Explorer 45 below L = 5.3 in the dusk magnetosphere on February 13, 1972. The particles were observed after a substorm that started at 1116 ± 0010 UT in the wake of an ssc at 0939 and that initiated a small magnetic storm with maximum Dst = 47 γ. The protons in the range between 1 and 40 keV exhibit strong dispersion effects in both energy and pitch angle. The electrons also show dispersion effects above 40 keV, while those below that energy are not observed until the satellite crosses the plasmapause. The observations are interpreted in terms of McIlwain's model electric field E3 and an injection boundary close to or coinciding with the plasmapause. Good quantitative agreement is obtained between the observed and the predicted proton energies and pitch angle dispersions and electron energy dispersions. In view of strong temporal fluctuations in the electron flux, no comparison in pitch angle dispersion of electrons is possible. The conclusion drawn is that in this event protons and electrons were injected at the onset of the substorm more or less instantaneously into the magnetosphere along or above the injection boundary and particles with energies below 10 keV were not convected in from the magnetospheric tail.

Magnetosphere inflation during four magnetic storms in 1965

Study of four magnetic storms in 1965, using Explorer 26 and ground observatory magnetic data, shows that both small and large magnetic storms develop through initial asymmetric inflation of the evening magnetosphere. Development times vary from 6 to 36 hours, and the development is sometimes interrupted. The inflating particles are injected as deep as L = 3 during the development phase in large storms but only to L = 4 in smaller storms. In the slow recovery phase there is a clear relation between the magnitude of the inflation and the location of the inflation belt. The April storm belt with 100-γ inflation at the beginning of slow recovery was at L = 3.7, whereas the February storm with 30-γ inflation was at L = 4.8. An unusual distortion of field direction on June 16 is interpreted as evidence for current flow along field lines at the onset of a substorm.

Solar and magnetic data, July to September, 1946, Mount Wilson Observatory

An immense complex bipolar sunspot‐group, Mount Wilson No. 8129, crossed the solar disk from July 19 to August 2. Its maximum area, measured at the United States Naval Observatory, was 3685 millionths of a solar hemisphere (4325 million square miles). A small flare was observed at Mount Wilson over the group on July 21 at 15h 11m GCT and a very intense one on July 25 from 16h 21m to 18h 30m.On July 26, a great magnetic storm began at 18h 47m (from Tucson magnetogram), more than 26 hours after the appearance of the bright flare of July 25 which showed on a spectroheliogram at 16h 21m but not on one at 16h 05m. An aurora, observed at Mount Wilson near midnight of July 26 (July 27d08h), and the small magnetic storm on July 29–30 also were probably associated with this immense group.

Solar and magnetic data, July to September, 1942, Mount Wilson Observatory

A small magnetic storm recorded on July 11–12 began when the bipolar sunspot‐group Mount Wilson No. 7455, which had developed rapidly on July 4, was four days past the central meridian. Another small magnetic storm on August 23–24 began when the active bipolar group No. 7478, which developed on August 22, had just crossed the central meridian. A third small magnetic storm occurred on September 11–12. Only a very small sunspot‐group was present on those dates.

Sitka Magnetic Observatory April to June, 1942

April 2—A small disturbance began gradually at about 08h GMT and continued until about 14h. A K‐index of 7 was recorded for the disturbance.April 4—A small magnetic storm began abruptly at 04h 38m GMT, April 4, and continued with large bays and superimposed short‐period vibrations until the close of April 4. K‐indices of 8 were recorded for three consecutive three‐hour intervals.

Sitka Magnetic Observatory, April to June, 1941

April 19—A short period of increased activity began gradually at 05h GMT, April 19. Maximum storminess occurred between 08h and 09h with decreased values of all elements. After 09h conditions slowly returned to normal.April 24–25—A small magnetic storm began abruptly at 08h 39m GMT, April 24. The activity increased gradually until about 15h and then began a gradual decrease. The disturbance consisted of large bays. After 15h a short‐period vibration was superimposed on the long‐period motion. After 08h, April 25, there was a slow return to normal. Ranges : D, 126′; H, 884 gammas; Z, 804 gammas.

Sitka Magnetic Observatory April to June, 1940

April 2–3—A small magnetic storm began gradually at about 17h GMT, April 2, and slowly increased to maximum values at 04h, April 3. During the major portion of the storm the traces moved with large regular periodic motion. After 10h, April 3, the storm gradually decreased. Ranges: D, 166′; H, 769 gammas; Z, 610 gammas.

A Naked-Eye Sunspot

A GIANT sunspot, easily visible to the naked eye, is crossing the sun's disk in latitude 17° north from October 5 until 18, the time of central meridian passage being October 11.9. The area of this spot on October 6, corrected for foreshortening, was nearly 2,500 millionths of the sun's hemisphere, and on October 8 its area measured 2,800 millionths, the measurements being made at Greenwich Observatory. The spot on the latter date extended in solar longitude for about 125,000 miles. Sunspots as large as this one are usually associated with terrestrial magnetic disturbances commencing about one day after the time of central meridian passage. On October 7, a small magnetic storm was recorded at the Greenwich magnetic observatory at Abinger, the disturbance reaching a maximum between 16h and 20h U.T. The ranges of the magnetic elements were: in declination 51' ; in horizontal force 200? ; and in vertical force 260?. It seems uncertain, however, whether this magnetic disturbance can be linked to the big sun-spot ; the most probable time of one related to the disturbed area of the sun containing this sunspot would be about October 12-13.

Auroral Display on February 3

REPORTS from various observers in Yorkshire and south Lancashire, who witnessed a display of the aurora borealis in the evening of February 3 last, have appeared in the Yorkshire Post. The aurora, which was evidently a striking phenomenon to those favoured with a clear sky, began about 18h 30m and lasted for an hour. One observer at Bradford noted a broad band of red passing overhead from east to west. On inquiry at the Royal Observatory, Greenwich, it was stated that a magnetic disturbance, which could be associated with this aurora, had been recorded at the Abinger magnetic station. The disturbance began suddenly on February 2 at 23J1 5m U.T., and within a few hours a range of 220y in horizontal force had been registered. The disturbance then died down, but was renewed between 17h and 22h on February 3; a typical movement (associated with auroras) in declination occurred about 19h with a range of f °, the range in horizontal force being 190y. At the commencement of the magnetic disturbance on Feb. 2d 23h, the centre of the large sunspot, reported on p. 228 of NATURE of February 6, was about 40° west of the sun's central meridian and the following extremity of the spot about 32° west. A statistical analysis made at Greenwich of big sunspots and associated magnetic storms shows that spots of the size of the recent one are associated, in about two out of three cases, with either a large or a small magnetic storm, occurring on the average about 1J days after the central meridian passage of the spot. It may be added that 27 days (or a solar synodic rotation) before the aurora on February 3, the Abinger magnetic traces at about 19|h on January 7 show a movement distinctive of auroral activity. An observer at the Saltburn Coastguard Station who saw the aurora on February 3 is reported by the Yorkshire Post as having stated that he “saw the lights a month ago”.

Provisional solar and magnetic character‐figures, Mount Wilson Observatory, January, February, and March, 1936

No magnetic storm in which the horizontal intensity varied by as much as 100 gammas was recorded in the first quarter of 1936. On February 14 at 9h 24m, G.M.T., a sudden increase inH of 9 gammas was followed by feeble disturbances in the midst of which H decreased by 20 gammas in 15 or 20 minutes beginning at 15h 15m. A fade‐out in high‐frequency radio reception was reported to have occurred at exactly that time.1 On February 16 at 10h 58m a sudden increase inH of 21 gammas was followed by a very small magnetic storm.