Onset Of Main Phase Research Articles

On positive and negative ionospheric storms

A study is made of the intense storms of March 13–15, 1989 ( Dst = −600nT), October 20–21, 1989 ( Dst = −266 nT) and April 1–2, 1973 ( Dst = −211 nT) in regards to the appearance of positive storm before the beginning of a geomagnetic disturbance in the mid-latitudes and the occurrence of strong negative phase at the equator. F2 region global structure response to the geomagnetic storms was studied using fo F2 data obtained during the storms from a global network of ionosonde stations. Investigated phenomena were only observed, on October 20, 1989, in only three of the nineteen plots representing ∼16% occurrence for the case study of March and October 1989 storms: the positive storms at Slough (54.4°N) and Uppsala (59.86°N) and a negative storm at Ouagadougou (12.4°N). These ionospheric storms were caused by the southward turning of Bz at ∼2100 UT on October 18 which got to a change in Bz of δBz = 12.2 nT at 2300 UT on October 18. In the case of the storm of April 1–2, 1973, the phenomena had ∼69% occurrence: the positive storms at Wakkanai (45.4°N), Akita (39.7°N), Kokubunji (35.7°N), Kiev (50.5°N) Sofia(42.7°N), Ottawa (45.4°N), Boulder (40.0°N) and Point Arguello (35.6°N) and a negative storm at Manila (14.7°N). These ionospheric storms appear to be caused by the southward turning of Bz at ∼1500 UT which got to a change in Bz of δBz = 13.6 nT at 1900 UT on March 31. The non explanation of these phenomena before now is because in the studies of ionospheric storms it is assumed that the beginning of any particular disturbance is defined by the onset of the magnetic storm. The use of sudden storm commencement (SSC) and main phase onset (MPO) for fixing the beginning of magnetic and ionospheric storms is fraught with problems that render a determination of the exact onset time difficult. The notion of onset of the magnetic storm as a prevailing idea restricted the geoeffectiveness of the solar wind to post onset time thereby foreclosing the explanation of any aspect of the morphology of ionospheric storms whose origin precede the onset reference time.

Role of neutral wind and storm time electric fields inferred from the storm time ionization distribution at low latitudes: in-situ measurements by Indian satellite SROSS-C2

Abstract. Recently, there has been a renewal of interest in the study of the effects of solar weather events on the ionization redistribution and irregularity generation. The observed changes at low and equatorial latitudes are rather complex and are noted to be a function of location, the time of the storm onset and its intensity, and various other characteristics of the geomagnetic storms triggered by solar weather events. At these latitudes, the effects of geomagnetic storms are basically due to (a) direct penetration of the magnetospheric electric fields to low latitudes, (b) development of disturbance dynamo, (c) changes in atmospheric neutral winds at ionospheric level and (d) changes in neutral composition triggered by the storm time atmospheric heating. In the present study an attempt is made to further understand some of the observed storm time effects in terms of storm time changes in zonal electric fields and meridional neutral winds. For this purpose, observations made by the Retarding Potential Analyzer (RPA) payload on board the Indian satellite SROSS-C2 are examined for four prominent geomagnetic storm events that occurred during the high solar activity period of 1997-2000. Available simultaneous observations, from the GPS satellite network, are also used. The daytime passes of SROSS-C2 have been selected to examine the redistribution of ionization in the equatorial ionization anomaly (EIA) region. In general, EIA is observed to be weakened 12-24 h after the main phase onset (MPO) of the storm. The storm time behaviour inferred by SROSS-C2 and the GPS satellite network during the geomagnetic storm of 13 November 1998, for which simultaneous observations are available, is found to be consistent. Storm time changes in the delay of received GPS signals are noted to be ~1-3 m, which is a significant component of the total delay observed on a quiet day. An attempt is made to identify and delineate the effects of a) meridional neutral winds, b) the development of the ring currents and c) the disturbance dynamo electric fields on the low latitude ionization distribution. The weakening of the EIA is noted to be primarily due to the decrease in the eastward electric fields driving the equatorial fountain during the daytime. The meridional neutral winds are also noted to play an important role in redistribution of ionization in the EIA region. The present results demonstrate that storm time latitudinal distribution of ionization in this region can be better understood by taking into account the meridional winds in addition to E×B drifts.

Broadband plasma waves observed in the polar cap boundary layer: Polar

Polar observations indicate the presence of intense broadband plasma waves nearly all of the time (96% occurrence frequency in this study) near the apogee of the Polar trajectory (∼6–8 RE). The region of wave activity bounds the dayside (0500 to 1800 LT) polar cap magnetic fields, and we thus call these waves polar cap boundary layer (PCBL) waves. The waves are spiky signals spanning a broad frequency range from ∼101 to 2 × 104 Hz. The waves have a rough power law spectral shape. The wave magnetic component has on average a ƒ−2.7 frequency dependence and appears to have an upper frequency cutoff of ∼(6–7) × 103 Hz, which is the electron cyclotron frequency. The electric component has on average a ƒ−2.2 frequency dependence and extends up to ∼2 × 104 Hz. The frequency dependences of the waves and the amplitude ratios of B′/E′ indicate a possible mixture of obliquely propagating electromagnetic whistler mode waves plus electrostatic waves. There are no clear intensity peaks in either the magnetic or electric spectra which can identify the plasma instability responsible for the generation of the PCBL waves. The wave character (spiky nature, frequency dependence and admixture of electromagnetic and electrostatic components) and intensity are quite similar to those of the low‐latitude boundary layer (LLBL) waves detected at and inside the low‐latitude dayside magnetopause. Because of the location of the PCBL waves just inside the polar cap magnetic field lines, it is natural to assume that these waves are occurring on the same magnetic field lines as the LLBL waves, but at lower altitudes. Because of the similar wave intensities at both locations and the occurrence at all local times, we rule out an ionospheric source. We also find a magnetosheath origin improbable. The most likely scenario is that the waves are locally generated by field‐aligned currents or current gradients. We find a strong relationship between the presence of ionospheric and magnetosheath ions and the waves near the noon sector. These waves may thus be responsible for ion heating observed near the cusp region. Antisunward convection of these freshly accelerated oxygen ions over the polar cap during intense wave events (occurring during southward Bz events) might lead to enhanced plasma sheet O+ population. For magnetic storm intervals this mechanism would lead to a natural delay between the main phase onset and the appearance of oxygen ions in the ring‐current.

Geoeffectiveness of three Wind magnetic clouds: A comparative study

We compare the large‐scale goemagnetic response to the three magnetic clouds observed by Wind in October 1995 (OCT95), May 1996 (MAY96), and January 1997 (JAN97), studying specifically storm and substorm activity, and other global effects due to untypically large and variable solar wind dynamic pressures. Since the temporal profiles of the interplanetary parameters of the three clouds resemble one another closely, the comparison is meaningful. Using the integrated Poynting flux into the magnetosphere as a rough measure of energy input into the magnetosphere, we find relative energy inputs to be OCT95: JAN97:MAY96 = 22:11:4, with most of the accumulation in the 3‐day periods occurring during passage of the Bz < 0 cloud phase. The peak Dst ring current indices, corrected for magnetopause currents, were in the ratio −138:−87:−38, and hence OCT95 caused a major, JAN97 a moderate, and MAY96 a weak storm. The empirical criterion derived from studies near solar maximum that a solar wind dawn‐dusk electric field ≥ 5mV m−1 lasting for at least 3 hours is necessary and sufficient to generate major storms does not hold for JAN97. Storm main phase onset coincides with cloud arrival in all three cases. The number of substorm onsets during the cloud periods were OCT95: JAN97:MAY96 = 5:3:2, with peak AL values in the ratio −1180:−1750:−570. The dayside magnetosphere was variably compressed, the largest amplitude of variation being on JAN97, where the dynamic pressure change spanned 2 orders of magnitude. MAY96 showed the least variation. The interaction of the individual clouds with the faster trailing flows had two major effects on the magnetosphere: (1) a compression of the cavity during passage of the Bz > 0 cloud phase and the leading edge of the fast stream; and (2) a weakening of the control of the cloud field on magnetosheath flow during the Bz > 0 cloud phase. In summary we find that under most of the aspects considered, OCT95 is the most geoeffective. The buffetting of the magnetospheric cavity by dynamic pressure changes was, however, strongest on JAN97. The profound differences in the magnetospheric response elicited by the clouds is found to be due to the amplitude, duration and rapidity of change of the relevant interplanetary parameters. At present, interplanetary monitors are indispensable for understanding the geomagnetic response to interplanetary structures.

Changes in the distant tail configuration during geomagnetic storms

Changes in the structure of the distant tail associated with geomagnetic storms are studied by using plasma and magnetic field data obtained from Geotail. Thirteen storm intervals between October 1993 and October 1994 are examined when the satellite was located in the distant tail between X = −83 RE and X = −210 RE. Geotail observed the magnetosheath during all storms including those when the satellite was located near the nominal tail axis. Assuming the flow direction in the magnetosheath to be parallel to the magnetotail axis, we estimated the dimension and the flux of the tail for the magnetopause crossing events. Systematic changes of the distant tail structure are found in association with the development of the storms. Before the main phase onset of the storms, when the Dst shows positive excursion, the enhanced solar wind pressure reduces the average radius of the distant tail to ∼23 RE as compared with the quiet time value, ∼31 RE. During the storm main phase the dimension of the tail is comparable to the quiet time value in spite of the high solar wind pressure. This is attributed to the enhanced magnetic flux in the tail in association with the southward interplanetary magnetic field (IMF) Bz. An average energy of ∼5 × 1015 J is stored also in the distant tail during the storm main phase, which is a comparable value to that stored in the midtail during an intense substorm growth phase reported in the previous studies. During storm time, when the Bz component of the magnetosheath field was larger compared with the By component, the average tail cross section has a north‐south elongated elliptical shape. The average tail for quiet time, however, was elongated toward the dawn‐dusk direction. By component was larger than Bz component in our quiet time data set. These observations imply that the anisotropy in the magnetosheath magnetic field pressure for different IMF orientation changes the shape of the tail, which was also shown in global MHD models. Changes in the average magnetosheath parameters (density, speed, direction) observed during the storms are found to be consistent with solar wind fluctuations expected from corotating interaction regions.

Global ionospheric effects of the October 1989 geomagnetic storm

Based on a large data base from 40 ionosonde stations distributed worldwide and 12 total electron content stations, a case study is made on the global behavior of ionospheric responses to the great magnetic storm of October 1989. The magnetic storm was triggered by a solar flare with the largest class of X13/4B and started with a sudden storm commencement (ssc) at 0917 UT on October 20. After the initial phase the storm underwent two periods of maximum activities in the following 2 days. Low‐latitude auroras were sighted and reported in widely separated areas in both northern and southern hemispheres. In response to these magnetic and auroral activities the ionosphere showed remarkable effects. Depending on the local time of ssc occurrence, the ionospheric response differed appreciably. Impressive changes were long‐lasting, large‐scale effects, such as the severe depressions of foF2 at higher latitudes, the temporary suppression of the equatorial anomaly and large horizontal gradients at certain latitudes. Also observed were positive storm effects of short duration during the post‐sunset period in response to the onset of both ssc and main phase of the magnetic storm. These two positive storm effects showed different patterns suggesting different causal mechanisms. In addition, global propagation of large‐scale traveling ionospheric disturbances (TIDs) was seen during 2 nights, identified by dramatic rises of h′F with periodic fluctuations. The equatorward propagation velocities of the TIDs varied between 330 m/s and 680 m/s for the east Asia region.

A study of low‐latitude VHF scintillations in relation to electric fields during magnetic storms

Nighttime VHF scintillations observed at Lunping (magnetic latitude 14.7°N) during four specific seasonal storms showed four different types of behaviour: Scintillation activity was found to be either inhibited or triggered during magnetic storms depending on the phase of the storm and its local time. A qualitative explanation for this behaviour is attempted in terms of storm time electric fields. The triggering of postmidnight scintillation activity during the recovery phase seems to be a result of penetration of the dusk‐dawn electric fields due to shielding charges developed in the ring current region,while inhibition of usual premidnight scintillation activity seems to be related to the ionospheric disturbance dynamo electric fields acting opposite to normal fields. One case where postsunset scintillation activity was triggered seems to be a result of penetration of dawn‐dusk convection electric field which developed following the main phase onset.

Storm effects of ionospheric total electron content (TEC) at low latitudes.

The changes in the ionospheric Total Electron Content (TEC) obtained by the Faraday rotation method at a chain of low latitude stations all in India (longitude about 75°E) are studied during a few selected geomagnetic storms mostly in winter of 1975-76. For the location Octacamund very near the dip equator in India, TEC data were available by the Group delay method also, and these correlated very well with the TEC data obtained by the Faraday rotation method.Large changes of TEC, both positive and negative, were observed at all locations, but these did not bear any clear relationships either with the specific phases (Main Phase Onset, Recovery) or with the magnitude of the geomagnetic storms. Often, large changes were observed even before the Main Onset. The fountain effect, in which a strong equatorial electrojet is associated with a large noon-time bite-out in equatorial f0F2 and a large equatorial anomaly (excess ionozation at the crests at about±30° dip angles) or, its reverse effect (a weak electrojet associated with a lack of f0F2 bite-out and a decreased equatorial anomaly), were seen very clearly in some instances during or even before a geomagnetic storm. However, in many other cases, the fountain effect was largely distorted.It is surmised that apart from electric field effects, neutral winds not only of polar origin but probably of low latitude origin are creating large dynamic upheavals in the lowlatitude ionospheric regions, often in a random fashion, not only during geomagnetic storms but even before or long time after the storm onsets. If true, the origin of such winds needs investigation.

Comparison of storm-time changes of ionospheric total electron content at nearby locations in mid-latitudes.

The changes in the total electron content (TEC) at some mid-latitude locations in the American zone are compared during nine geomagnetc storms which are comparatively clean (clear-cut single main phase). It is found that TEC changes are sometimes related to sudden storm commencement (SSC) and sometimes to main phase onset (MPO) and the changes are sometimes positive and sometimes negative with no obvious relationship either with season or with any particular phase of the geomagnetic storm. It is concluded that there is no direct cause-effect relationship between ionospheric changes and changes of geomagnetic field as such and the only thing common between these is their common source viz, impact of interplanetary plasma blobs while subsequent evolutions are independent and unrelated.

Interplanetary field and plasma during initial phase of geomagnetic storms.

Twenty-three geomagnetic storm events during 1966 to 1970 were studied by using simultaneous interplanetary magnetic field and plasma parameters. Explorer 33 and 35 field and plasma data were analyzed on large-scale (hourly) and small-scale (3 min.) during the time interval coincident with the initial phase of the geomagnetic storms. The solar-ecliptic Bz component turns southward at the end of the initial phase, thus triggering the main phase decrease in Dst geomagnetic field. The By component also shows large fluctuations along with Bz. When there are no clear changes in the Bz component, the By shows abrupt changes at the main phase onset. On the small-scale, behavior of the magnetic field and electric field were studied in detail for the three events; it is found that the field fluctuations in By, Bz and Ey and Ez are present in the initial phase. In the large-scale, the behavior field remains quiet because the small-scale variations are averaged out. It appears that large as well as small time scale fluctuations in the interplanetary field and plasma help to alter the internal electromagnetic state of the magnetosphere so that a ring current could causing a geomagnetic storm decrease.

Observations of ionospheric storms at low latitudes and their correlation with magnetic field changes near the magnetic equator

Eight major magnetic storm periods occurring under sunspot maximum conditions (1968–1969) have been investigated using total electron content N T , maximum electron density N m and magnetic field data taken at low latitudes. Near Hong Kong (lat. 22.2°N, dip 30°) the main phase onset (MPO) of a storm occurring during the daytime was sometimes preceded by a slight increase and often followed by a sharp decrease in N m and N t , changes in N m being usually the most significant. The earlier the occurrence of the MPO after local midnight the more pronounced was an observed irregular build-up of ionisation after sunrise. Abrupt changes of the horizontal component of the geomagnetic field near the magnetic equator were often followed 2–3 hr later by similar sign changes in N m near Hong Kong and opposite sign changes in N m at Manila (10° lat. nearer the Equator). These results imply that during storms there are current variations induced in the electrojet region and the corresponding eastward electric field which uplifts ionisation at the magnetic equator also changes. It appears that during the main phases of the storms there were often reductions in the intensity of this eastward electric field (even reversals on some occasions) and the development of the equatorial anomaly was consequently retarded. During the northern summer a large day-to-day variability has been observed, even on magnetically quiet days, of the effect of the electrojet at the magnetic equator and of the development of the equatorial anomaly. Storms occurring during this summer period have not been included.

A study of the relationship between geo-magnetic storms and ionospheric disturbances at mid-latitudes

The problem of the ionospheric disturbances associated with geomagnetic storms is examined with the goal of searching for a relationship between the time-developments of the two phenomena. Faraday rotation measurements of total electron content ( N T ) are used to monitor the ionospheric F-region at a mid-latitude site, while a variety of geomagnetic parameters are examined as possible ways of following the geomagnetic variations. The ionospheric and geomagnetic data taken during 28 individual storms from 1967 to 1969 are used to search for a predictive scheme which can be tested using data from 17 storms in 1970. The specific aim is to find the geomagnetic parameter whose time-development can best forecast whether or not the ionospheric response will include an initial positive phase prior to the normally extended period of F-region depletions. Correlations between N T and the geomagnetic indices K p , and equatorial Dst(H) prove to be wholly inadequate. The local times of main-phase-onset (MPO) determined from the equatorial Dst(H) indices as well as from local horizontal component data, also prove to be unsatisfactory. The best correlations are obtained using local measurements of the total geomagnetic field ( F). These results show that a storm commencement (SC) will produce an enhancement in n t during the afternoon period following the SC unless there is an intervening post-midnight period with a strong depression of the geomagnetic field. Operationally this is taken to be a depression in F of at least 100γ near 03:00 LT

Changes in the distribution of low-energy trapped protons associated with the April 17, 1965, magnetic storm

The absolute intensity of geomagnetically trapped protons in the energy ranges 0.52 ≤ Ep ≤ 4.0 Mev and 0.90 ≤ Ep ≤ 1.8 Mev has been measured with the solid-state proton detector on the University of Iowa low-altitude (initial apogee 2502 km, perigee 527 km) high-latitude (inclination 81°) satellite Injun 4 for the period from March 1 to May 31, 1965. A study of the temporal variations of these fluxes associated with the April 17, 1965, magnetic storm (sc 1313 UT April 17, main phase onset ∼0200 UT April 18) shows a general redistribution of these protons for L ≳ 2.5 and all sampled |B| ranges, which persisted for at least 36 days after the storm. The effect of the sudden commencement was a general depression in the intensities and a hardening of the energy spectrums, although the intensities recovered to their prestorm level during the initial phase. The major redistribution was apparently initiated by the polar substorm that began at ∼0620 UT on April 18 and continued long after (to 1100 UT) the substorm had subsided (∼0800 UT). During the recovery phase, a secondary peak developed in the intensity profile at L ∼ 3.5 for 0.52-Mev protons that had no counterpart at this energy at the equator (L. R. Davis, private communication, 1971). No such peak was observed for 0.9-Mev protons. The over-all effect of the storm on the steady-state distribution was a nonadiabatic one with an increase in intensities at L ≲ 3 and a decrease at L ≳ 3. The loss and/or gain of particles appears to be fractionally the same at all |B| values sampled here. The post-storm dependence of the spectral parameter E0 on L generally follows the relation E0 ∝ L−3, although the prestorm dependence cannot be described by a simple function at all L values. The appearance of a secondary peak and the behavior of the prestorm and post-storm spectrums are in qualitative agreement with the predictions of the bimodal diffusion model of Theodoridis et al. [1969], whereby the solar wind is the source of the observed protons.