Auroral Breakup Research Articles

Vertical evolution of auroral acceleration at substorm onset

Abstract. This study describes the onset process of auroral substorms in connection with the vertical evolution of auroral particle acceleration, on the basis of auroral kilometric radiation (AKR) dynamics. We show that the auroral acceleration process at substorm onset basically consists of two stages: (1) appearance/intensification of a low-altitude acceleration region at 4000–5000 km accompanied by initial brightening and (2) breakout of high-altitude field-aligned acceleration above the pre-existing low-altitude acceleration region at 6000–12 000 km, which is followed by auroral breakup and poleward expansion. It is also revealed that this two-stage evolution of auroral acceleration corresponds to the two-step reinforcement of field-aligned current.

Timing and localization of ionospheric signatures associated with substorm expansion phase onset

In this paper, we present case studies of the optical and magnetic signatures of the characteristics of the first minute of substorm expansion phase onset observed in the ionosphere. We find that for two isolated substorms, the onset of magnetic pulsations in the 24–96 s period wavelet band are colocated in time and space with the formation and development of small‐scale optical undulations along the most equatorward preexisting auroral arc prior to auroral breakup. These undulations undergo an inverse spatial cascade into vortices prior to the release of the westward traveling surge. We also present a case study of a multiple activation substorm, whereby discrete onsets of ULF wave power above a predetermined quiet time threshold are shown to be associated with specific optical intensifications and brightenings. Moreover, in the multiple activation substorm event, we show that neither the formation of the small‐scale undulations nor the formation of similar structures along a north–south aligned arc is sufficient to produce auroral breakup associated with expansion phase onset. It is only ∼10 min after these two disparate activation regions initiate that auroral breakup and the subsequent formation of a westward traveling surge occur. We discuss the implications of these results in terms of the triggering mechanisms likely to be occurring during these specific events.

A state‐of‐the‐art picture of substorm‐associated evolution of the near‐Earth magnetotail obtained from superposed epoch analysis

We have obtained a state‐of‐the‐art picture of substorm‐associated evolution of the near‐Earth magnetotail and the inner magnetosphere for understanding the substorm triggering mechanism. We performed superposed epoch analysis of Geotail, Polar, and GOES data with 2‐min resolution, utilizing a total of 3787 substorms for each of which auroral breakup was determined from Polar UVI or IMAGE FUV auroral imager data. The decrease of the north‐south magnetic field associated with plasmoids and the initial total pressure decrease suggest that the magnetic reconnection first occurs in the premidnight tail, on average, at X ∼ −16 to −20 RE at least 2 min before auroral onset. The magnetic reconnection site is located near the tailward edge of a region of considerably taillike magnetic field lines and intense cross‐tail current, which extends from X ∼ −5 to −20 RE in the premidnight sector. Then the plasmoid substantially evolves tailward of X ∼ −20 RE immediately after onset. Almost simultaneously with the magnetic reconnection, the dipolarization begins first at X ∼ −7 to −10 RE 2 min before onset. The dipolarization region then expands tailward as well as in the dawn‐dusk directions and earthward. We find that the total pressure generally enhances in association with the dipolarization, with the contribution of high‐energy particles. Also, energy release is more significant between the regions of the magnetic reconnection and the initial dipolarization. The present results will be helpful as a reference guide to developing the overall picture of magnetotail evolution and studying the causal relationship between the magnetic reconnection and the dipolarization as well as detailed mechanisms of each of the two processes on the basis of multispacecraft observations.

Toward adapted time‐dependent magnetospheric models: A simple approach based on tuning the standard model

We suggest and test a simple procedure to adapt a magnetic field model by fitting it to observations made simultaneously by several spacecraft. This is done by varying input parameters of a standard model (T96) to find the best fit to the observed field at each time step. As a result we obtain a time‐dependent model which can be used for evaluating the quality of the standard model and of the mapping at any particular time, to navigate in the magnetosphere and reproduce its variable configuration during large‐scale dynamical events. This procedure was tested using observations made by five Time History of Events and Macroscale Interactions during Substorms (THEMIS) and other complementary (e.g., GOES) spacecraft during the tail season of THEMIS mission (January–March 2008), for which a simplest version of the adapted model was routinely calculated and has been made publicly available. We also use the proton isotropic boundaries observed by low‐altitude NOAA spacecraft for independent evaluation of the obtained field models. We found that in quiet conditions deviations of ionospheric footprints between standard and adapted models are generally small (within 1° of latitude), whereas during substorms they may be as large as several degrees, because of stretching and dipolarizations of magnetospheric configuration. We found that the variable tilt of the tail current sheet, partly caused by variations of nonradial component of the solar wind flow, is an additional important factor influencing the modeling result and the mapping quality. By analyzing the adapted models constructed at the time of auroral breakup onset, we conclude that this simple approach is not yet sufficiently accurate to evaluate the source distance in the magnetotail.

Wave dispersion and the discrete aurora: New constraints derived from high‐speed imagery

An analysis of multiscale observations of a substorm auroral breakup are presented which clarify the role of wave dispersion in the formation of elemental (<100 m) auroral structure. At coarse resolution (all‐sky white light camera, 1 frame/s), observations fit the established substorm morphology—namely, arc brightening, formation of spatial distortions, and breakup into multiple “rayed” structures. At fine‐scale resolution (electron multiplying charge‐coupled device [EMCCD] camera, 9‐degree field of view, prompt emission filter, 30 frames/s), an entirely different type of coherence is observed. The “arc,” as identified at lower resolution, is observed to be a dynamic structure composed of bifurcating elemental arcs that propagate outward from the center of an “arc packet.” This dynamic process is well captured in time‐brightness histories (keograms) along a cut bisecting the structure. The observations are interpreted with respect to theoretical predictions for inertial Alfvén wave dispersion. Specifically, the arc packets are interpreted as the B⊥ projection of the parallel electric field within the Alfvén resonant cone. Prebreakup observations are found to be qualitatively consistent with this model. However, some difficulties are encountered for the more active postbreakup period. The article includes a discussion of perspective considerations in interpreting small‐scale auroral features; in particular, it is shown that the “rayed” appearance of discrete breakup aurora is, in fact, a consequence of sharply kinked sheets viewed obliquely.

Longitudinal association between magnetotail reconnection and auroral breakup based on Geotail and Polar observations

The dawn–dusk locations of reconnection in the near‐earth magnetotail at the time of isolated auroral breakup are studied to clarify whether breakup is always accompanied by reconnection. The near‐earth reconnection is identified by tailward plasma flows faster than 200 km/s with southward magnetic field. We first identified 66 breakups in the Polar ultraviolet imager observations of the nightside polar ionosphere. We then studied tailward flows during breakups using Geotail in situ observations of the plasma sheet between 25 and 31 RE down the tail. It was found that the dawn–dusk (Y) locations of relatively fast (≥400 km/s) tailward flows were associated with breakup magnetic local time (MLT) by a regression line of YAGSM = −5.7 × (MLT + 0.6) RE with a correlation coefficient of 0.8. Most tailward flows were observed within 5 RE of the modeled Y locations, where tailward flows occurred in 88% of the 26 cases of breakups between 22 and 0 MLT. It is thus inferred that in most cases, breakup is accompanied by tailward flow near the breakup MLT with its dawn–dusk dimension ∼10 RE. There were only two events without tailward flows in the region where flows have been expected. These two events were an earthward flow event and a traveling compression region event, which are not inconsistent with the initiation of the near‐earth reconnection. Auroral breakup is thus likely to always be accompanied by near‐earth reconnection near breakup MLT. It is also inferred that reconnection and breakup occur simultaneously within a few minutes, assuming a time delay between reconnection onset and the arrival of tailward flows at satellite locations.

Modes and characteristics of low‐frequency MHD waves in the near‐Earth magnetotail prior to dipolarization: Fitting method

Spacecraft observations indicate that low‐frequency (0.006–0.025 Hz) fluctuations of the magnetic field appear a few min prior to a substorm‐associated dipolarization. These fluctuations are examined on the basis of the linear magnetohydrodynamic (MHD) fluid theory. We propose a “low‐frequency fitting method” for identifying the characteristics of the MHD waves, such as the mode, the wavevector, and the frequency. Using the magnetic field and the ion velocity data, the frequency in the plasma rest frame is obtained by removing the Doppler shift. The fitting method takes the inhomogeneity of the ambient magnetic field into account, so that the parameter which is equivalent to the sum of the field line curvature and the gradient scale length of the ambient magnetic field is obtained as an output. We applied the method to a selected dipolarization event in which Geotail remained in the vicinity of the magnetic equator and also in the same magnetic local time as the onset region of an auroral breakup. The low‐frequency fluctuations were detected from ∼4 min before the local dipolarization onset. What has been found are as follows: (1) the parallel magnetic field fluctuations had a strong correlation with the perpendicular ion velocity fluctuations, which indicates that the observed waves were explained as magnetosonic modes. The slow magnetosonic wave was identified ∼3 min before the dipolarization onset, whereas the fast magnetosonic wave was identified ∼1.5 min before the dipolarization onset. This fast wave was estimated to be propagating tailward with a phase velocity of 400 km/s and a period of 70 s in the plasma frame. (2) The perpendicular fluctuations of the magnetic field and the ion velocity were only weakly correlated, which indicates the presence of a mode that cannot be captured by the present method. Using the variance analysis, we show that this mode is likely to be a drift mode, which had almost zero frequency in the plasma frame and was propagating duskward together with the plasma bulk flow. A possible interpretation of the observed waves is briefly discussed with a relevance to previously proposed substorm initiation models. Particularly, the drift wave can be interpreted as a linear stage of the ballooning instability.

Multispacecraft and ground‐based observations of substorm timing and activations: Two case studies

Two case studies are performed to investigate substorm timing and activations based on Double Star TC1, Cluster, Polar, IMAGE, LANL geostationary satellites and ground‐based geomagnetic field measurements. In both events, an earthward flow associated with plasma sheet thinning is measured by Cluster 8–10 min ahead of the auroral breakup. A couple of minutes after the breakup, either TC1 at ∼X‐10 RE first detects plasma sheet expansion and then the LANL satellites near the midnight measure energetic electron injections at geostationary orbit or the LANL satellites first measure the electron injections and then TC1 detects the plasma sheet expansion. More than about 20 min later, Cluster at X∼16 RE and Polar (at higher latitude) successively observe plasma sheet expansion. The open magnetic flux of the polar cap, Ψ, is found to continually increase during the early substorm phase and then to rapidly fall when the IMF turns northward. When Ψ reaches its minimum value, bright and broad auroral activities start to decrease. Tailward progression of the magnetic dipolarization and a poleward expansion of auroral bulges are shown to closely map to one another. These results suggest that substorm activations start in the midtail before ground onset and then move earthward, which leads to an expansion onset in the near‐Earth tail around X∼ ‐(8–9) RE. After onset, the activations progress both earthward and tailward. Substorm onset is possibly related to plasma sheet reconnection of close field lines, while tail lobe reconnection of open field lines release more energy to support the full expansion of the substorm. In a fully developed expansion phase, an initial dipolarization in the near‐Earth may eventually evolve to enable disruption of the cross‐tail current over a wide region of the magnetotail.

Auroral radio emission and absorption of medium frequency radio waves observed in Iceland

Abstract In order to study the generation and propagation processes of MF auroral radio emissions (referred to as auroral roar and MF burst) in the polar ionosphere, an Auroral Radio Spectrograph (ARS) system was installed at Husafell station in Iceland (invariant latitude: 65.3°). Data analysis of man-made transmissions also provides useful information for the ionosphere study as well as an investigation of auroral radio emissions since the propagation character of MF radio waves changes depending on electron-neutral collisions in the bottomside ionosphere. Thus, ionospheric absorption is examined in comparison with the solar zenith angle and auroral phenomena. The results indicate that the ARS data can be used to detect ionization occurring at distant regions. In late 2006, the ARS detected one auroral roar and twoMF bursts, which were identified as left-handed polarized waves. Results of data analysis, including other auroral data and particle spectra observed by the DMSP satellite, suggest that the MF bursts are generated by electrons with an average energy of several keV associated with auroral breakup. On the other hand, the auroral roar is generated as upper hybrid waves by relatively low-energy electrons over the observation site and propagates downward, being converted into L-O mode electromagnetic waves.

Fine structure of auroras during auroral breakup according to the ground-based and satellite observations

The dynamics and fine structure of auroras before and during 60 auroral breakups, including pseudobreakups and breakups at moderate and high auroral activity, have been studied using the developed method for processing television images. The IMAGE and POLAR satellite and simultaneous ground images of auroras, ground magnetic data, and measurements of IMF and solar wind plasma parameters have been analyzed. The signatures that can be precursors of breakup have been found out in the auroral dynamics and morphology in the spatial—temporal vicinity of breakup. The morphological characteristics of auroral structures have been analyzed statistically. The directions of motion of weak subvisual structures have been determined. The velocities of motion of such structures are presented. The relation of the initial auroral arc bright-ening during breakups and pseudobreakups to the beginning of magnetic activation and formation of rayed structures has been analyzed.

The magnetospheric boundary motion during the main phase of the moderate magnetic storm (case study)

As a result of ICME interaction with magnetosphere the negative sudden impulse Si and substorm event were followed. The simultaneous plasma and magnetic field measurements near the magnetospheric boundary (INTERBALL-1), in the nearest magnetotail (GOES 8, 9), in the middle tail (GEOTAIL), in solar wind (WIND), and ground based data were compared to trace the time sequence of events. It was shown that negative sudden impulse Si was observed by ground stations and at geosynchronous altitudes. The character of magnetic field variations that were observed by GOES 9 permit to suggest that compression/expansion wave was propagated inside the magnetosphere. The time sequence of events observed during substorm development pointed that substorm has begun from auroral breakup. The cross current disruption in the nearest tail and neutral line formation in the middle tail occurred almost simultaneously, but some time after auroral breakup. It is possible that both processes associated with releasing of stored energy during substorm were occurred independently in this substorm event. By using an empirical magnetopause model [Shue et al., 1998. Magnetopause location under extreme solar wind conditions. J. Geophys. Res. 103, 17691], the amplitude of magnetopause motion was evaluated. During substorm evolution the amplitude of the magnetopause motion was ∼ 2 –3 Re. The excitation of Kelvin-Helmholtz instability can explain only a few magnetopause crossings.

The Relationship between the Magnetic Cloud Boundary Layer and the Substorm Expansion Phase

The magnetic cloud boundary layer (BL) is a disturbance structure that is located between the magnetic cloud and the ambient solar wind. In this study, we statistically an- alyze the characteristics of the magnetic field Bz component (in GSM coordinates) inside the magnetic cloud boundary layers as well as the relationship between the magnetic cloud boundary layers and the magnetospheric substorms based on 35 typical BLs observed by Wind from 1995 to 2006. It is found that the magnetic field Bz components are more turbu- lent inside the BLs than those inside the adjacent sheath regions and the magnetic clouds. The substorm onsets are identified by the auroral breakups that are the most reliable sub- storm indicators by using the Polar UVI image data. The UVI data are available only for 17 BLs. The statistical analysis indicated that 9 of the 17 events triggered the substorms when BLs crossed the magnetosphere and that the southward field in the adjacent sheath region is a necessary condition for these triggering events. In addition, the SF-type BLs, which are named by their features of the Bz components inside the BLs and adjacent sheath regions, can easily trigger the substorms during their passage of the magnetosphere. SF-type BLs are characterized by sustained strong southward magnetic fields persisting for at least 30 minutes in the adjacent sheath regions and at least one change in the polarity of the Bz component inside the BL. In this study, 7 out of 8 such SF-type BL events triggered the sub- storm expansion phase, suggesting that the SF-type BLs are another important interplanetary disturbance source of substorms.

A plasma bulk motion in the midnight magnetosphere during auroral breakup inferred from all-sky image and magnetic field observations at geosynchronous altitudes

Auroral events that occurred on January 24, 1986 in central Canada were recorded by an all-sky TV imager. During these events, auroral breakup was confined to a region between two foot points of neighboring geosynchronous satellites, GOES5 and GOES6. We examined field line signatures at satellite locations in unique station distributions and concluded that field line observation indicated plasma motion in the equatorial plane. The plasma motion showed an earthward compression combined with bifurcation (duskward or dawnward displacement in dusk/dawn sectors). In addition, we were able to infer an elliptical circulation of plasmas in the equatorial plane at Pi2 periods. Appearance in opposite rotation beside the auroral region indicated excitation of surface waves. We were able to show that auroral breakups occurred at a meridian of bifurcation. We suggest that a high plasma pressure region occurring tailward of geosynchronous altitudes may drive those plasma motions.

Simultaneous DMSP, all-sky camera, and IMAGE FUV observations of the brightening arc at a substorm pseudo-breakup

Abstract Auroral particles, field-aligned currents, and plasma convections in the vicinity of the brightening arc at substorm onset are still not well understood, since it is very rare to have conjugate satellite measurements above the brightening arc. In this paper, we investigate the characteristics of auroral particles and fields associated with the brightening arc at a pseudo-onset of substorm on October 31, 2000, using ground all-sky TV images, IMAGE FUV auroral images, and particle, magnetic field, and plasma flow data obtained by the DMSP F12 satellite. The arc brightening at Tixie (66.0°MLAT), Russia, occurred at 1004 UT (18.75 MLT) coincident with a coherent Pi 2 pulsation at midlatitudes and with the DMSP crossing above the arc. The brightening arc did not develop on a global scale, indicating that this event is a pseudo auroral breakup, which occurred ≈16 min before the major substorm expansion onset. IMAGE auroral images indicate that the longitude of the brightening center was ≈2.5 h nightside of Tixie. The DMSP data show that the precipitating particles associated with the brightening arc correspond to an electron inverted-V structure at the equatorward edge of the electron precipitation region. The arc was located in the energetic (>1 keV) ion precipitation region, near the equatorward boundary of the upward region 1 field-aligned current, and at the peak of the sunward convection velocity. These facts indicate that the brightening arc at duskside of the onset local time was located in the inner plasma sheet at the inner edge of the region 1 current source in the sunward convection region.

Global manifestations of a substorm onset observed by a multi-satellite and ground station network

Abstract. With a favorable constellation of spacecraft and ground stations, a study is made on the global manifestations of a substorm onset. The onset occurred simultaneously and conjugately in both hemispheres, confirmed by observations of the auroral breakup from IMAGE FUV-WIC and a sudden intensification of a westward electrojet from ground-based magnetometers. Concurrently with the onset, field-aligned and Hall currents in the auroral ionosphere are observed by CHAMP, which are consistent with the signature of a Harang discontinuity. Immediately after the onset a magnetic field dipolarization is clearly observed by Double Star TC-1, located near the central magnetotail and subsequently, by the Cluster quartet. The observations can be explained by a dawnward propagation of the substorm current wedge at a speed of about 300 km/s.

How can we solve the problem of substorms? (A review)

The works in the alternative direction of magnetospheric studies are reviewed. In contrast to the traditional approach, where the basis process is magnetic field line reconnection, transformation of kinetic energy into electromagnetic one at the bow shock front is the basis process in the proposed approach. It has been indicated that this new paradigm makes it possible to overcome the main difficulties that remained within the scope of the previous paradigm. It has been briefly demonstrated how several following processes and phenomena are explained within the scope of the new approach: (1) transformation of the solar wind kinetic energy into the electromagnetic energy; (2) electromagnetic energy transfer into the magnetosphere; (3) organization of the system of bulk currents, formation of field-aligned currents from the magnetosphere, and compatibility of these currents with the ionospheric current systems; (4) shape, value, and dynamics of the particle precipitation auroral regions; and (5) substorm expansion (auroral breakup). Other possibilities of the new approach and paradigm replacement consequences are briefly considered.

Further study of flickering auroral roar emission: 1. South Pole observations

Hughes and LaBelle (2001) reported a single example of a new geophysical phenomenon: ∼10 Hz modulation of auroral radio emissions near twice the auroral ionospheric electron gyrofrequency. They called this phenomenon flickering auroral roar and suggested that it is related to flickering aurora, which results from ∼10 Hz modulation of the precipitating auroral electrons. Observations at South Pole Station during 2003 using a new high‐bandwidth receiving system have yielded 10 examples of flickering roar emissions. Although 10 examples is still a small number for statistics, these observations considerably extend previous knowledge of this phenomenon which was based on a single example. On the basis of the 2003 South Pole data set, flickering auroral roar accounts for only about 1–2% of auroral roar in number of seconds, but ∼20% of auroral roar events have some flickering feature. The observed modulation frequencies range from ∼3 to 30 Hz. The 10‐ to 20‐Hz modulations, which correspond to about 65% of the time when flickering roar occurs, are much more common than the higher‐frequency 20‐ to 30‐Hz modulations, which correspond to about 20% of the time when flickering roar occurs. These frequencies compare favorably with optical observations of auroral emissions, rocket observations of electron flux modulations, and modeling results. If these frequencies correspond to the oxygen gyrofrequency where electrons and ion cyclotron waves interact, the 3‐ to 30‐Hz frequency range would imply sources at altitudes of 1500–10,500 km. Six of the ten flickering roar examples occurred during substorm expansion phase as defined from local magnetometer data, in contrast to flickering aurora, which is usually observed after the onset of auroral breakup.

Longitudinal dependence of characteristics of low-latitude Pi2 pulsations observed at Kakioka and Hermanus

AbstractWe statistically investigated longitudinal dependence of characteristics of low-latitude Pi2 pulsations to find the longitudinal structure of the plasmaspheric cavity mode. We used the geomagnetic field data from two ground stations, Kakioka (27.2° geomagnetic latitude, 208.5° geomagnetic longitude) and Hermanus (—33.9° geomagnetic latitude, 82.2° geomagnetic longitude), and auroral image data acquired by the ultraviolet imager onboard the Polar satellite for the period of December 4, 1996 to March 3, 1997. Our findings include the following: (1) Pi2 amplitude is the largest around the magnetic local time of the auroral breakup site and decreases away from it; (2) when a nightside Pi2 pulsation has large amplitude, a dayside Pi2 pulsation can be observed with a similar waveform; (3) Pi2 pulsations generally have no clear phase differences (mean phase difference of 3.3°) between Kakioka and Hermanus, except for some events; and (4) the phase difference is independent on AMLT (difference of magnetic local time between a station and the auroral breakup). These observations suggest that the plasmaspheric cavity mode can be excited globally with a very small value of the azimuthal wave number(m≈0).

Statistical study of the substorm onset: its dependence on solar wind parameters and solar illumination

Abstract. Based on 1829 well-defined substorm onsets in the Northern Hemisphere, observed during a 2-year period by the FUV Imager on board the IMAGE spacecraft, a statistical study is performed. From the combination of solar wind parameter observations by ACE and magnetic field observations by the low altitude satellite CHAMP, the location of auroral breakups in response to solar illumination and solar coupling parameters are studied. Furthermore, the correspondence of the onset location with prominent large-scale field-aligned currents and electrojets are investigated. Solar illumination and the related ionospheric conductivity have significant effects on the most probable substorm onset latitude and local time. In sunlight, substorm onsets tend to occur 1h earlier in local time and 1.5° more poleward than in darkness. The solar wind input, represented by the merging electric field, integrated over 1h prior to the substorm, correlates well with the latitude of the breakup. Most poleward latitudes of the onsets are found to range around 73° magnetic latitude during very quiet times. Field-aligned and Hall currents observed concurrently with the onset are consistent with the signature of a westward travelling surge evolving out of the Harang discontinuity. The observations suggest that the ionospheric conductivity has an influence on the location of the precipitating energetic electron which causes the auroral break-up signature. Keywords. Ionosphere (Auroral ionosphere) – Magnetospheric Physics (Current systems; Magnetosphereionosphere interactions)

Report from scientific sessions at the 35 th COSPAR scientific assembly, Paris 2004

As a result of ICME interaction with magnetosphere the negative sudden impulse Si and substorm event were followed. The simultaneous plasma and magnetic field measurements near the magnetospheric boundary (INTERBALL-1), in the nearest magnetotail (GOES 8, 9), in the middle tail (GEOTAIL), in solar wind (WIND), and ground based data were compared to trace the time sequence of events. It was shown that negative sudden impulse Si was observed by ground stations and at geosynchronous altitudes. The character of magnetic field variations that were observed by GOES 9 permit to suggest that compression/expansion wave was propagated inside the magnetosphere. The time sequence of events observed during substorm development pointed that substorm has begun from auroral breakup. The cross current disruption in the nearest tail and neutral line formation in the middle tail occurred almost simultaneously, but some time after auroral breakup. It is possible that both processes associated with releasing of stored energy during substorm were occurred independently in this substorm event. By using an empirical magnetopause model [Shue et al., 1998. Magnetopause location under extreme solar wind conditions. J. Geophys. Res. 103, 17691], the amplitude of magnetopause motion was evaluated. During substorm evolution the amplitude of the magnetopause motion was ∼2–3 Re. The excitation of Kelvin-Helmholtz instability can explain only a few magnetopause crossings.