Interval Of Northward Interplanetary Magnetic Field Research Articles

The Effect of IMF Fluctuations on the Subsolar Magnetopause Position: A Study Using a Global MHD Model

AbstractDuring southward interplanetary magnetic field (IMF) conditions, the subsolar magnetopause moves inward even under steady solar wind pressure. This is generally referred to as “erosion.” Empirical and computational studies have been conducted to quantify the effect of magnetopause erosion; however, they generally use steady state IMF conditions. Solar wind conditions are not usually constant for very long, especially during such conditions as are present in high‐speed streams, which typically have large fluctuations in the IMF. These IMF fluctuations can occur on a range of times scales and amplitudes, resulting in a series of northward and southward IMF intervals imposed on the magnetosphere. We have simulated the effect of such IMF variations on the magnetopause position using a global magnetohydrodynamic model. We find that there are both time lag and hysteresis‐like effects on the motion of the magnetopause due to the variation in the magnitude of the southward IMF and that the instantaneous position of the magnetopause can be significantly different from what one would expect from steady state conditions.

Diagnosing the Role of Alfvén Waves in Magnetosphere‐Ionosphere Coupling: Swarm Observations of Large Amplitude Nonstationary Magnetic Perturbations During an Interval of Northward IMF

AbstractHigh‐resolution multispacecraft Swarm data are used to examine magnetosphere‐ionosphere coupling during a period of northward interplanetary magnetic field (IMF) on 31 May 2014. The observations reveal a prevalence of unexpectedly large amplitude (>100 nT) and time‐varying magnetic perturbations during the polar passes, with especially large amplitude magnetic perturbations being associated with large‐scale downward field‐aligned currents. Differences between the magnetic field measurements sampled at 50 Hz from Swarm A and C, approximately 10 s apart along track, and the correspondence between the observed electric and magnetic fields at 16 samples per second, provide significant evidence for an important role for Alfvén waves in magnetosphere‐ionosphere coupling even during northward IMF conditions. Spectral comparison between the wave E‐ and B‐fields reveals a frequency‐dependent phase difference and amplitude ratio consistent with interference between incident and reflected Alfvén waves. At low frequencies, the E/B ratio is in phase with an amplitude determined by the Pedersen conductance. At higher frequencies, the amplitude and phase change as a function of frequency in good agreement with an ionospheric Alfvén resonator model including Pedersen conductance effects. Indeed, within this Alfvén wave incidence, reflection, and interference paradigm, even quasi‐static field‐aligned currents might be reasonably interpreted as very low frequency (ω → 0) Alfvén waves. Overall, our results not only indicate the importance of Alfvén waves for magnetosphere‐ionosphere coupling but also demonstrate a method for using Swarm data for the innovative experimental diagnosis of Pedersen conductance from low‐Earth orbit satellite measurements.

What Happens Before a Southward IMF Turning Reaches the Magnetopause?

AbstractPrevious observations have shown a ∼10–15 min time delay in the ionospheric response to solar wind directional discontinuities marked by either southward or northward interplanetary magnetic field (IMF) turnings. We have studied one southward IMF turning observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) and GOES in the dayside magnetosphere. Using a global MHD model, we have reproduced the magnetopause motion in this event. We find that the observed delay in the ground response can be completely explained by deceleration of the directional discontinuity in the subsolar magnetosheath. We show that the speed of the discontinuity significantly decreases in the vicinity of the magnetopause where the magnetic barrier formed during the previous northward IMF interval. The southward turning can reach the magnetopause only after complete disruption of the magnetic barrier. The disruption or dissipation occurs via magnetosheath reconnection, as confirmed by high‐speed jets in the magnetosheath. The magnetopause moves sunward as the directional discontinuity transits the magnetosheath. This sunward motion is followed by the earthward motion when the discontinuity strikes the magnetopause and magnetopause reconnection begins.

First results using TWINS‐derived ion temperature boundary conditions in CRCM

AbstractWe have integrated dynamic, spatiotemporally resolved ion temperature boundary conditions into the Comprehensive Ring Current Model (CRCM), which are based on 2‐D equatorial maps derived from the Two Wide‐Angle Imaging Neutral‐Atom Spectrometers (TWINS) energetic neutral atom (ENA) data. The high‐speed stream‐driven event on 22 July 2009 is simulated and compared against an identical simulation using a statistically derived boundary condition model. ENA‐derived temperatures allow users to include event‐specific observations associated with a dynamic plasma sheet. This method also provides temperatures in the important region between geosynchronous orbit and the plasma sheet, a region which existing empirical models exclude. We find that the spatial and energy distributions of ring current flux and pressure have sensitive dependence on boundary conditions during this event. The coupling of boundary conditions to the time history of the convection field strength also plays an important role by throttling the influence of the boundary plasma on the inner magnetosphere. Simulated moments and spectra from our simulations are compared with remotely imaged ion temperatures from TWINS and also in situ energy spectra and temperature moments from Time History of Events and Macroscale Interactions during Substorms‐D. Storm time dusk‐dawn asymmetries consistent with observational data, such as Zhang et al. (2006), are reproduced well when CRCM is provided with the event‐specific boundary model. A hot localized structure observed by TWINS at geosynchronous midnight during a strong northward interplanetary magnetic field interval is also reproduced with this boundary model, whereas the empirical boundary model fails to yield this feature.

An MHD simulation study of the dynamics of the 8–9 March 2008 CIR‐/HSS‐driven geomagnetic storm

AbstractWe have carried out a global magnetohydrodynamic (MHD) simulation of a geomagnetic storm initiated by a corotating interaction region followed by a high‐speed solar wind (HSS) stream that occurred on 8–9 March 2008. The event began with the arrival of a corotating interaction region (CIR) at ~0720 UT on 8 March. The stream interface arrived at Earth at ~1830 UT on 8 March, and the arrival of a second density enhancement (a second CIR) at ~0140 UT on 9 March resulted in the main phase of the storm, with a peak Dst of −97 nT at 0600 UT on 9 March. Our MHD simulation of the event, spanning the interval of 0400 UT on 8 March to 0800 UT on 9 March, shows that the arrival of the first CIR changes the configuration of the magnetotail, and that after a strong substorm at ~1230 UT on 8 March, the tail evolves into a churning state in which the magnetic topology and flow structure of the magnetotail are never steady. In addition, we find that increases in ring current energy density show a nearly one‐to‐one correspondence to periods of VxBz > 0 (southward interplanetary magnetic field (IMF)). More importantly, we find that the ring current energy density in the MHD simulation shows a nearly linear response to increases in solar wind dynamic pressure, but only for the northward IMF intervals during the initial phase of the event, from 0400 UT to 1800 UT on 8 March.

Sources of electron pitch angle anisotropy in the magnetotail plasma sheet

We survey the properties of electron pitch angle distributions in the magnetotail plasma sheet at a distance between 15 and 19 RE from the Earth, using data from the Plasma Electron and Current Experiment (PEACE) instrument. We limit our survey to those pitch angle distributions measured when the interplanetary magnetic field (IMF) had been steadily northward or steadily southward for the previous 3 h. We find that, at sub‐keV energies, the plasma sheet electron pitch angle distribution has an anisotropy such that there is a higher differential energy flux of electrons in the (anti‐) field‐aligned directions. Fitting the measured pitch angle distributions with both a single and two component kappa distribution reveals that this anisotropy is the result of the presence of a second, cold, component of electrons that is observed more often than not, and occurs during both the northward and southward IMF intervals. We present evidence that suggests the cold electron component has an ionospheric, rather than magnetosheath, source and is linked to the large‐scale field‐aligned current systems that couple the magnetosphere and ionosphere.

Substorms under northward interplanetary magnetic field: Statistical study

It was recently noted that substorms can occur even under prolonged northward interplanetary magnetic field (IMF) conditions. Based on the substorm list obtained from the IMAGE spacecraft, we perform a statistical study on the features of substorms during northward IMF interval. The strength of the substorm is represented by the AL index decrease and the total intensity of the auroral bulge. Four main features have been found as follows: (1) Most substorms occur soon after a southward IMF, and intense substorms are more likely to occur for short duration of northward IMF period (2 ~ 5 h), whereas no intense substorms occur after prolonged northward IMF condition. (2) There is a positive correlation between the strength of the substorm and the two solar wind parameters (the IMF |By| and the solar wind dynamic pressure Pd). (3) The average strength of the substorms during the storm period is much larger than that of the substorms during the period without storm. Meanwhile, nearly all strong storm time substorms occurred either during the intense storm period, or during the late main phase or the early recovery phase of the storm. (4) About half of substorms, either an increase or a decrease in the solar wind dynamic pressure, are found within 30 min preceding each onset time. Such features indicate that the energy stored in the magnetotail during a previous southward IMF period is the main energy source for substorms under northward IMF condition, especially for intense substorms, and both the IMF |By| and the solar wind dynamic pressure play an important role in the energy accumulation during the northward IMF period.

Turbulence in a global magnetohydrodynamic simulation of the Earth's magnetosphere during northward and southward interplanetary magnetic field

Abstract. We report the results of MHD simulations of Earth's magnetosphere for idealized steady solar wind plasma and interplanetary magnetic field (IMF) conditions. The simulations feature purely northward and southward magnetic fields and were designed to study turbulence in the magnetotail plasma sheet. We found that the power spectral densities (PSDs) for both northward and southward IMF had the characteristics of turbulent flow. In both cases, the PSDs showed the three scale ranges expected from theory: the energy-containing scale, the inertial range, and the dissipative range. The results were generally consistent with in-situ observations and theoretical predictions. While the two cases studied, northward and southward IMF, had some similar characteristics, there were significant differences as well. For southward IMF, localized reconnection was the main energy source for the turbulence. For northward IMF, remnant reconnection contributed to driving the turbulence. Boundary waves may also have contributed. In both cases, the PSD slopes had spatial distributions in the dissipative range that reflected the pattern of resistive dissipation. For southward IMF there was a trend toward steeper slopes in the dissipative range with distance down the tail. For northward IMF there was a marked dusk-dawn asymmetry with steeper slopes on the dusk side of the tail. The inertial scale PSDs had a dusk-dawn symmetry during the northward IMF interval with steeper slopes on the dawn side. This asymmetry was not found in the distribution of inertial range slopes for southward IMF. The inertial range PSD slopes were clustered around values close to the theoretical expectation for both northward and southward IMF. In the dissipative range, however, the slopes were broadly distributed and the median values were significantly different, consistent with a different distribution of resistivity.

Successive substorm expansions during a period of prolonged northward interplanetary magnetic field

[1] We have studied successive substorm expansions that occurred in spite of a period of prolonged northward interplanetary magnetic field (IMF). After the northward IMF persisted for almost ∼19 h, a series of 11 very weak to moderate substorm expansions occurred during the following 6-h interval of northward IMF on 19 January 1998, separated from each other by a few tens of minutes. Most of these substorm expansions did not accompany significant changes of the solar wind and the IMF. As for typical substorms that occur under southward IMF conditions, spacecraft and ground-based instruments observed auroral breakups and enhancements of the westward auroral electrojet and large-scale convection in the ionosphere, together with plasmoids and dipolarizations in the magnetotail, in association with the substorm expansion onsets. These signatures suggest that the mechanism of substorm expansion onsets is the same as for typical substorms, even under prolonged northward IMF conditions. We suggest that the large IMF ∣By∣ affects large-scale convection and energy accumulation in the magnetotail and that the enhanced convection due to the large IMF ∣By∣ effect and substorm expansions can be an important factor for the promotion of succeeding substorm expansion onsets.

Evidence that solar wind fluctuations substantially affect the strength of dayside ionospheric convection

Ionospheric convection is occasionally observed to be substantially enhanced even when the interplanetary magnetic field (IMF) is not strongly southward and the IMF By is not large. Such enhanced convection flows tend to exhibit large oscillations with ∼10–30 min periodicity. We have considered the solar wind characteristics that lead to these oscillatory convection enhancements. We have used an extensive set of Sondrestrom radar observations of ionospheric convection within the dayside polar cap. We find that IMF ULF power is closely associated with the strength of dayside convection. Convection flows during periods of large north–south IMF fluctuations are observed to be as strong as for steady and large southward IMF periods. Enhanced convection is also observed during northward IMF intervals when the interplanetary magnetic field exhibits high ULF power. We find that ULF power enhances the convection strength, independent of an observed direct effect from the solar wind speed. These observations thus suggest that IMF ULF fluctuations can significantly influence ionospheric convection. Therefore, in addition to the well‐established contributions from the direction and magnitude of the IMF and the solar wind dynamic pressure, ULF fluctuations may also be an important contributor to coupling of the solar wind to the magnetosphere‐ionosphere system. We speculate that resonance between IMF fluctuations and natural magnetospheric oscillation frequencies or magnetopause boundary oscillations might be responsible for the connection between ionospheric convection and IMF ULF power. We have also found evidence for a connection between the ULF power in the solar wind dynamic pressure and the strength of convection.

Plasma plume circulation and impact in an MHD substorm

We investigate the fate of a plasmaspheric plume generated by a discrete period of southward interplanetary magnetic field (IMF) to assess its contribution to plasma sheet and ring current pressure and compare with that for other sources. We use test particle motions in Lyon‐Fedder‐Mobarry (LFM) global circulation model fields. The inner magnetosphere is simulated with the Comprehensive Ring Current Model (CRCM) model of Fok and Wolf, driven by the transpolar potential developed by the LFM magnetosphere. A variant of the Ober plasmasphere model is embedded within the models and driven by them. Global circulation is stimulated by a period of southward IMF embedded within a long interval of northward IMF. This leads to the production of a well‐defined plasmaspheric plume, enhancing the plasma density sunward of the plasmasphere. Test particles are launched with the properties of plasmaspheric ions on the L = 6.6 RE shell and weighted with densities as specified by the Ober model, as it responds to convection imposed by CRCM. Particles are tracked until they are lost from the system downstream or into the atmosphere, using the Delcourt full equations of motion, implemented for finite element fields. Results are compared with earlier computations of polar and auroral wind outflows. The plume produces an enhanced flow of plasma ∼10 times the normal polar wind global fluence. However, we find that most of the “plasmaspheric wind” is lost from the magnetosphere such that its contribution to the ring current energy density is comparable to that of the normal polar wind for this type of event.

The existence and properties of the distant magnetotail during 32 hours of strongly northward interplanetary magnetic field

We report observations made by the Wind spacecraft in the distant magnetotail on 22–24 October 2003 when the interplanetary magnetic field (IMF) was strongly northward for more than 32 h. A well‐defined magnetotail was observed down to at least XGSM = −125 RE even after 32 h of strongly northward IMF conditions. However, the observed tail properties were strikingly different from the typical magnetotail. Although the closed field line plasma sheet was observed throughout the northward IMF interval, the usual mantle/lobe disappeared one hour after the northward IMF front reached the frontside magnetopause. Instead, open field lines were observed populated by field‐aligned solar wind strahl electrons as well as plasma sheet electrons predominantly at 90° pitch angle. In essence, these observations provide evidence for reconnection between the IMF and a previously closed nightside cold dense plasma sheet during long periods of northward IMF, after poleward‐of‐the‐cusp reconnection has depleted all open lobe field lines.

Penetration electric fields: A Volland–Stern approach

This paper reformulates the Volland–Stern model, separating contributions from corotation and convection to predict electric field penetration of the inner magnetosphere using data from the Advanced Composition Explorer (ACE) satellite. In the absence of shielding, the model electric field is E VS= Φ PC/2 L Y R E, where Φ PC is the polar cap potential and 2 L Y R E is the width of the magnetosphere along the dawn–dusk meridian. Φ PC is estimated from the interplanetary electric field (IEF) and the dynamic pressure of the solar wind ( P SW); values of L Y were approximated using P SW and simple force-balance considerations. ACE measurements on 16–17 April 2002 were then used to calculate E VS for comparison with the eastward electric field component ( E Jφ ) detected by the incoherent scatter radar at Jicamarca, Peru. While the interplanetary magnetic field (IMF) was southward, the model predicted observed ratios of E VS/IEF. During intervals of northward IMF, E Jφ turned westward suggesting that a northward IMF B Z system of field-aligned currents affected the electrodynamics of the dayside ionosphere on rapid time scales.

Magnetosphere preconditioning under northward IMF: Evidence from the study of coronal mass ejection and corotating interaction region geoeffectiveness

Motivated by recent observations and simulations of the formation of a cold and dense plasma sheet in the tail of the magnetosphere under northward interplanetary magnetic field (IMF) and of the direct influence of the plasma sheet density on the ring current strength, this paper aims at (1) highlighting how the coupling of these effects may lead to a preconditioning of the magnetosphere under northward IMF and (2) performing first tests of the validity of this hypothesis. We have analyzed superposed epoch time series of various parameters to investigate the response of the magnetosphere (as indicated by the Dst index) to the passage of coronal mass ejections (CMEs) and corotating interaction regions (CIRs). We first focused on the difference between the measured Dst signature and that predicted by a semiempirical Dst model. For both CME‐ and CIR‐driven storms the superposed epoch results show that the model Dst predictions tend to underestimate the actual storm strength (by up to 10–30%) for events that are preceded by a substantial interval of northward IMF, as opposed to those with no such preceding northward IMF. We also analyzed Los Alamos geosynchronous spacecraft data for these events. The average density and temperature measured at storm onset are substantially higher and slightly lower, respectively, for the cases with preceding northward IMF intervals. These results suggest that solar wind structures may be more geoeffective if preceded by a northward IMF interval and they are consistent with the hypothesis of a preconditioning by a cold, dense plasma sheet. A colder and denser plasma sheet may lead to a stronger ring current when that plasma is convected inward during the main phase of an ensuing storm.

Cross-Scale Coupling Within Rolled-Up MHD-Scale Vortices and Its Effect on Large Scale Plasma Mixing Across the Magnetospheric Boundary

Kelvin-Helmholtz Instability (KHI) is an MHD-scale instability that grows in a velocity shear layer such as the low-latitude boundary layer of the magnetosphere. KHI is driven unstable when a velocity shear is strong enough to overcome the stabilization effect of magnetic field. When the shear is significantly strong, vortices in the nonlinear stage of KHI is so rolled-up as to situate magnetospheric plasma outward of the magnetosheath plasma and vice versa. The big question is if such highly rolled-up vortices contribute significantly to the plasma transport across the boundary and to the filling of the plasma sheet by cool magnetosheath component, which is observed under northward Interplanetary Magnetic Field (IMF) condition. Here we review our recent results from two-fluid simulations of MHD-scale KHI with finite electron inertia taken into account. The results indicate that there is coupling between the MHD-scale dynamics and electron-scale dynamics in the rolled-up stage of the vortices. While the details differ depending on the initial magnetic geometry, the general conclusion is that there is significant modification of the MHD-scale vortex flow pattern via coupling to the micro-physics. The kick-back from the parasitic micro-physics enhances highly the potential for large-scale plasma mixing of the parent MHD-scale vortices, which is prohibited by definition in ideal-MHD. We also review our recent 3-D MHD simulation results indicating that KHI vortex can indeed roll-up in the magnetotail-flank situation despite the strong stabilization by the lobe magnetic field. These results encouraged us to search for evidence of rolled-up vortices in the Cluster formation flying observations. As reviewed in this paper, a nice event was found during northward IMF interval. This interval is when the plasma transport via large scale reconnection becomes less efficient. The finding supports the argument that KHI is playing some role in transporting solar wind into the magnetosphere when the normal mode of transport cannot dominate.

Pulsed flows observed during an interval of prolonged northward IMF

Abstract. On the 22 December 2002 the interplanetary magnetic field (IMF) was directed northwards for more than 12h. The Northern and Southern Hemisphere SuperDARN radars were used to study global high-latitude convection during this interval, complemented by data from the ACE and DMSP F13 spacecraft. The relative magnitudes of the IMF By and Bz components varied during this period. When the magnitude of the By component was comparable with or dominated the Bz component, signatures of simultaneous low-latitude and lobe reconnection were observed. Specifically two "standard" merging cells were observed in both hemispheres. In the Northern Hemisphere a high-latitude lobe cell was observed within the dusk merging cell, and there was also evidence of a narrow viscous cell located equatorward of this lobe cell. We observed the ionospheric signatures of flux transfer events (FTEs) in both the Northern and Southern Hemispheres, occurring with a periodicity of ~15min. In the Northern Hemisphere the FTEs were associated with a stepwise equatorward progression of the equatorward boundary of radar backscatter on the dayside. When the IMF Bz component was predominantly greater than the IMF By component, we observed a four-cell convection pattern in the Northern Hemisphere, with pulses of reverse reconnection and an associated stepwise poleward retraction of the equatorward boundary of radar backscatter occurring every ~25min. These observations are consistent with pulsed lobe reconnection occurring in both hemispheres, closing open flux and adding closed flux to the dayside magnetopause. So, during this northward IMF interval the location of the sites of reconnection between the IMF and the Earth's magnetosphere, and thus the form of reconnection process, varied with changing IMF conditions. However, the reconnection remained pulsed, with lobe-only reconnection having a significantly longer periodicity compared with simultaneous lobe and low-latitude reconnection.

Steady reconnection during intervals of northward IMF: Implications for magnetosheath properties

This study examines the location of the reconnection site on the magnetopause for conditions of northward interplanetary magnetic field (IMF) and its implications for properties of the magnetosheath. Ion distribution functions from the Toroidal Imaging Mass Angle Spectrometer (TIMAS) instrument on board the Polar spacecraft during passes through the dayside cusp region when the IMF was steady and northward are used. Cutoff velocities are determined from these distributions and the distance from the Polar spacecraft to the site of reconnection is estimated. A magnetospheric magnetic field model is used to map these determined distances to a location on the magnetopause where reconnection is believed to have occurred. Nearly all of these reconnection sites lie in places where the magnetosheath flow is estimated to be super‐Alfvénic (using hydrodynamic theory and an analytic magnetosheath magnetic field model). However, there exist strict constraints on the speed of the ambient magnetosheath flow at the reconnection site, in order for this particular type of particle distribution to have been observed by TIMAS. Different physical models are discussed, including the possibility of nonstationary reconnection sites and the existence of a plasma depletion layer which significantly increases the magnetosheath Alfvén speed close to the magnetopause. From the observations and mapped reconnection locations, we estimate statistically how much the average ion density must decrease (and the magnetic field must increase) in the plasma depletion layer to be consistent with the cusp region observations. The resulting range of values is consistent with the theoretical estimates of Zwan and Wolf [1976] (k ≥ 2).

Solar–wind–magnetosphere–ionosphere interactions in the Earth's plasma environment

The properties of the Earth’s coupled magnetosphere-ionosphere system are dominated by its interaction with the solar wind plasma, mediated by magnetic reconnection processes at the magnetopause interface. In this paper we focus on recent progress and remaining questions concerning the nature of these interactions, how they depend on the orientation of the interplanetary magnetic field (IMF), and the consequences which follow for internal dynamics. The progress we report, on flux transfer events and substorms during intervals of southward IMF, and magnetopause and tail processes during intervals of northward IMF, emphasise the great diagnostic power of combined in situ and remote sensing observations from space and on the ground.

Increase of the tail plasma content during the northward interplanetary magnetic field intervals: Case studies

We have analyzed the structure of the magnetotail current sheet during the northward interplanetary magnetic field (IMF) intervals, using the method devised by Sergeev et al. [1998]. Case studies suggest that systematic variations of the current sheet thickness are seen not only during substorm activities but also during a building up phase of the cold dense plasma sheet when the IMF is northward and the magnetosphere is geomagnetically quiet. It is further found that during such northward IMF intervals the plasma “vertical content,” namely the product N0λ with the plasma density N0 at the plasma sheet center and the characteristic thickness λ, showed an order‐of‐magnitude increase within several hours. We estimate plasma transport rate during the northward IMF to be as much as 1026/s, which cannot be neglected in comparison with that during the southward IMF (∼1027 s−1).

Sunward polar cap convection

This is a study of the conditions for which there is sunward convection in the central polar cap. The study of convection direction uses digital ionosonde measurements from two central polar cap stations (Resolute Bay, corrected geomagnetic (CG) latitude 83.3°N, and Eureka, CG latitude 88.67°N) and presents statistical results about pure sunward convection events. The sunward convection events in the central cap region are observed only when the interplanetary magnetic field (IMF) Bz is >2 nT. For weak northward IMF (<3 nT), sunward convection events are confined to the region near the noon cusp. For strong northward IMF conditions (>3 nT), sunward convection occurs at all local times, indicating the expansion of reverse convection to most of the central polar cap. There are very notable IMF By effects on the pattern of sunward convection. For IMF By positive the axis of the pattern of reverse convection appears to rotate toward the postnoon, whereas for IMF By negative the axis is rotated toward the prenoon. These axis orientations are compatible with the expected locations of lobe cell merging. Another significant IMF By effect is that the duration of sunward convection intervals when IMF By is negative is about the same as the duration of the northward IMF intervals, whereas for positive IMF By the sunward convection intervals usually last only for a short duration (∼1 hour). However, if the northward IMF interval is much longer than 1 hour, there is a sequence of these ∼l‐hour sunward convection events. The convection pattern during northward IMF appears to be compatible with a four‐cell form.