Earth's Outer Radiation Research Articles

Megavolt Parallel Potentials Arising from Double-Layer Streams in the Earth’s Outer Radiation Belt

Huge numbers of double layers carrying electric fields parallel to the local magnetic field line have been observed on the Van Allen probes in connection with in situ relativistic electron acceleration in the Earth's outer radiation belt. For one case with adequate high time resolution data, 7000 double layers were observed in an interval of 1 min to produce a 230,000 V net parallel potential drop crossing the spacecraft. Lower resolution data show that this event lasted for 6 min and that more than 1,000,000 volts of net parallel potential crossed the spacecraft during this time. A double layer traverses the length of a magnetic field line in about 15 s and the orbital motion of the spacecraft perpendicular to the magnetic field was about 700 km during this 6 min interval. Thus, the instantaneous parallel potential along a single magnetic field line was the order of tens of kilovolts. Electrons on the field line might experience many such potential steps in their lifetimes to accelerate them to energies where they serve as the seed population for relativistic acceleration by coherent, large amplitude whistler mode waves. Because the double-layer speed of 3100 km/s is the order of the electron acoustic speed (and not the ion acoustic speed) of a 25 eV plasma, the double layers may result from a new electron acoustic mode. Acceleration mechanisms involving double layers may also be important in planetary radiation belts such as Jupiter, Saturn, Uranus, and Neptune, in the solar corona during flares, and in astrophysical objects.

Rapid local acceleration of relativistic radiation-belt electrons by magnetospheric chorus

Recent analysis of satellite data obtained during the 9 October 2012 geomagnetic storm identified the development of peaks in electron phase space density, which are compelling evidence for local electron acceleration in the heart of the outer radiation belt, but are inconsistent with acceleration by inward radial diffusive transport. However, the precise physical mechanism responsible for the acceleration on 9 October was not identified. Previous modelling has indicated that a magnetospheric electromagnetic emission known as chorus could be a potential candidate for local electron acceleration, but a definitive resolution of the importance of chorus for radiation-belt acceleration was not possible because of limitations in the energy range and resolution of previous electron observations and the lack of a dynamic global wave model. Here we report high-resolution electron observations obtained during the 9 October storm and demonstrate, using a two-dimensional simulation performed with a recently developed time-varying data-driven model, that chorus scattering explains the temporal evolution of both the energy and angular distribution of the observed relativistic electron flux increase. Our detailed modelling demonstrates the remarkable efficiency of wave acceleration in the Earth's outer radiation belt, and the results presented have potential application to Jupiter, Saturn and other magnetized astrophysical objects.

Duskside relativistic electron precipitation as measured by SAMPEX: A statistical survey

Evidence for duskside relativistic electron precipitation (DREP) within the Earth's outer radiation belt has historically been seen in a few sets of high altitude balloon data (MAXIS, MINIS, INTERBOA), and in satellite data. We present statistical evidence that the relativistic electron precipitation events from the outer radiation belt with e‐folding energies > 0.5 MeV are concentrated in the dusk‐to‐midnight sector, based on a survey of data collected by the SAMPEX satellite from 1992 to 2004. A correlation between spectral hardness and duskside MLT is observed in our sample, the largest studied to date. Out of 9380 precipitation events within the bounce loss cone, 1048 are observed to have exponentially falling spectra with e‐folding energies above 0.5 MeV (“hard events”) and 1648 events below 0.2 MeV. Of the hard events, 81% occur within 12 h to 24 h MLT, compared to only 37% of events having e‐folding energies below 0.2 MeV. With microbursts removed from this softer population the percentage of duskside events rises to 46%. The hard events occur at slightly elevated levels of geomagnetic activity (Ap and Dst) relative to softer nonmicroburst events, but these correlations are much weaker than for microbursts. The hard events are observed to peak in occurrence at L ~ 5.5, significantly higher than nonmicroburst softer events, even though the opposite might be expected from compression of the magnetosphere due to the more negative average Dst of the hard events. The hard events are most prevalent during the declining phase of the 11 year solar cycle.

Significant loss of energetic electrons at the heart of the outer radiation belt during weak magnetic storms

AbstractFor various reasons, the Earth's outer radiation belt often exhibits dramatic and sudden increases or decreases in the observed particle flux. In this paper, we report three dropout events of energetic electrons observed by multiple spacecraft while traveling across the outer radiation belt. The three events were first identified based on observations of a significant dropout in the >2 MeV electron flux at geosynchronous orbit. Subsequently, for each event, we analyzed the energetic electron data obtained near the magnetic equator by THEMIS spacecraft to determine the responses of the entire outer radiation belt. Our analysis is mainly based on the electron fluxes measured at energies of 52 keV, 203 keV, and 719 keV, and on the phase space densities estimated for the first adiabatic invariant μ values of 100 MeV/G, 200 MeV/G, and 300 MeV/G. The main shared feature among the three events is that while, for the lowest energy, sources from the convection and/or particle injections of plasma sheet electrons dominate over losses, the higher energies exhibit a dramatic dropout effect that penetrates deeply into L ~ 4.5 – 5. In terms of the phase space density, a similar dropout effect is clearly seen for the μ values of 200 MeV/G and 300 MeV/G, while the convection effect and/or injections dominates for μ = 100 MeV/G. What is astonishing about this dropout phenomenon is that the three events are all associated with only very weak magnetic storms with a SYM‐H minimum of ‐40 nT or larger. This implies that a significant loss of electrons deep inside the outer radiation belt can occur even during a very weak magnetic storm. Low‐altitude observations of electrons by NOAA POES satellites indicate no significant atmospheric precipitation due to strong diffusion. Our simulations with various conditions suggest that radial diffusion effect in combination with the magnetopause shadowing are responsible for the observed dropouts to a large extent for all of the three events, although the contribution by the weak atmospheric precipitation that might have been missed by the NOAA POES observations can be non‐negligible.

Long‐term loss and re‐formation of the outer radiation belt

AbstractThe Earth's outer radiation belt is known to vary often and significantly on various time scales. In this study, we have used the data of various instruments onboard the THEMIS spacecraft to study long‐term changes of the outer radiation belt electrons around the year 2009. We find that the entire outer belt became extremely weak for nearly a year and was practically lost a few times, each time lasting ~20 days up to ~2 months, before eventually re‐forming. This was revealed at a wide energy range from several tens of keV to up to 719 keV, which was covered by the THEMIS spacecraft measurements. The loss of the outer belt was associated with extremely weak solar wind conditions, i.e., low interplanetary magnetic field magnitude and slow solar wind speed. In particular, this set greatly reduced magnetospheric convection and/or injections for a prolonged time interval, which led to a large expansion of the plasmasphere, even beyond geosynchronous altitude and thus invading the majority of the typical outer belt territory for the same prolonged time interval. Consequently, preexisting electrons inside the plasmasphere had enough time to be lost into the atmosphere gradually over a time scale of several days without being supplied with fresh electrons from the plasma sheet under the same reduced convection and/or injections. Plasmaspheric hiss waves with an amplitude of up to a few tens of pT persisted to exist during the gradual decay periods, implying that they are likely responsible for the continual loss of the electrons inside the plasmasphere. A complete re‐formation of the outer belt to full intensity was then realized over an interval of a few months. During the re‐formation process, the magnetospheric convection and/or injections increased, which led to a gradual increase of whistler chorus wave activity, contraction of the plasmasphere, and supply of the plasma sheet electrons at high L shells. This set first an outward increasing profile of the phase space density, which eventually developed into a profile with a peak at low L of ~5 over a time scale of 1–2 days. In this latter stage, a local acceleration at low L shells is found to be clearly needed although the radial diffusion process can contribute to some extent, in particular, for particles with a low first adiabatic invariant value.

On the storm‐time evolution of relativistic electron phase space density in Earth's outer radiation belt

We report on internal, magnetospheric processes related to markedly different storm‐time responses of phase space density (PSD) in invariant coordinates corresponding to equatorially mirroring, relativistic electrons in Earth's outer radiation belt. Two storms are studied in detail, selected from a database of 53 events (Dstmin < −40 nT) during the THEMIS era thus far (December 2007–August 2012). These storms are well covered by a number of in situ THEMIS spacecraft and complemented by additional ground‐based and in situ observatories, and they epitomize the divergent behaviors that the outer radiation belt electrons can exhibit during active periods, even during otherwise similar Dst and auroral electrojet (AE) profiles. From our statistical results with the full database, the changes in the radial profile peak in PSD reveal notably consistent behavior with prior studies: 58% of geomagnetic storms resulted in PSD peak enhancements, 17% resulted in PSD peak depletions, and 25% resulted in no significant change in the PSD peak after the storm. For the two case studies, we examined the PSD at multiple equatorial locations (using THEMIS), trapped and precipitating fluxes from low‐Earth orbit (using POES), and chorus, hiss, EMIC, and ULF waves (using THEMIS spacecraft, ground observatories, and the GOES spacecraft). We show that (1) peaks in PSD were collocated with observed chorus waves outside of the plasmapause during the most active periods of the PSD‐enhancing storm but not during the PSD‐depleting storm, providing evidence for the importance of local acceleration by wave‐particle interactions with chorus; (2) outer belt dropouts occurred following solar wind pressure enhancements during both storms and were consistent with losses from magnetopause shadowing and subsequent outward radial transport; during the PSD‐enhancing storm, this revealed how the outer belt can replenish itself seemingly independently of the remnant of the pre‐existing belt leftover after a dropout, which in this case resulted in a double‐peaked outer belt distribution; (3) slow decay in PSD was associated with corresponding locations in L* and enhanced wave amplitudes of plasmaspheric hiss; (4) precipitation loss associated with wave‐particle interactions with hiss and EMIC waves appeared to be significantly more important during the PSD‐depleting storm than the PSD‐enhancing storm; and (5) PSD transport during the recovery phase of both storms and throughout the PSD‐enhancing storm was consistent with ULF‐wave‐driven radial diffusion away from maxima in PSD; this indicates the importance of ULF waves in redistributing outer belt PSD after local acceleration occurs. We conclude that these source, transport, and loss processes, individually well characterized by previous studies, do indeed appear to act in concert, leading to predominance of local acceleration in one case and loss in another. These processes can therefore conspire toward optimal source or loss of outer belt electrons under suitable external drivers, and the conditions resulting in wave growth for these acceleration and loss mechanisms are therefore an important area of future research.

The spectral extent of chorus in the off‐equatorial magnetosphere

Magnetospheric chorus waves are a major driver of acceleration and loss in the Earth's outer electron radiation belt. The spectral extent of chorus is a key parameter in quantifying the global effect of chorus on energetic particle populations by determining the range of resonant electron energies. However, statistics of spectral properties are sparse, particularly in the off‐equatorial magnetosphere. We use a database of chorus observations from the Polar spacecraft to generate statistics on the normalized chorus frequency (with the respect to the minimum field line gyrofrequency, Ωmin) as a function of magnetic local time (MLT) (0 < MLT < 24), L‐shell (3 < L < 11), and magnetic latitude (|λ|< 65°). We find that, on average, the chorus spectrum peaks in the range of 0.1–0.4 Ωmin, varying significantly with λ, R0 and MLT. The normalized chorus peak frequency is found to decrease with increasing R0, and decreases with increasing latitude below ∼ 25°. When fit to a Gaussian spectral model, lower band chorus is found to have a bandwidth < 0.1 Ωmin, which is narrower than assumed in most diffusion models. Diffusion coefficients calculated using the University of California at Los Angeles (UCLA) Full Diffusion Code show that wave‐particle interactions on the nightside are highly sensitive to both the peak frequency and bandwidth of chorus, yet on the dayside scattering is mostly sensitive to the peak frequency as a result of the wider latitudinal extent of the waves. We also find that fitting a Gaussian to the logarithm of the spectrum reduces fit errors by over 60%, indicating that inclusion of arbitrary spectral forms may improve the accuracy of wave models within radiation belt simulations.

Acceleration of radiation belts electrons by oblique chorus waves

The redistribution of energy during the recovery phase of geomagnetic storms related to the acceleration of electrons in the Earth's outer radiation belt by cyclotron‐resonant chorus waves is an important and challenging topic of magnetospheric plasma physics. An approximate analytical formulation of energy diffusion coefficients is derived in this paper, on the basis of a quasi‐linear formalism valid for large enough bandwidths or for successive random scatter by uncorrelated waves of different frequencies and moderate average amplitudes. We make use of chorus wave parameterizations derived from CLUSTER measurements to show that oblique whistler waves can significantly increase the energy diffusion rate of small pitch angle electrons on the dayside. On the other hand, the energization rate of the more numerous high pitch angle electrons is typically reduced by a factor of 2 on the dayside, while it remains nearly unchanged on the nightside where high‐intensity waves are less oblique. Besides, lifetimes are strongly reduced on the dayside, which could also impact the long‐term time‐integrated acceleration rates of injected electrons. Comparison between the analytical formulas and full numerical results demonstrates a good agreement and provides new scaling laws as a function of whistler mean frequency, plasma density and particle energy. It is also suggested that the enhancement of energy diffusion of low energy electrons (<100 keV) at small pitch angles with oblique waves could result in an intensification of wave growth at latitudes higher than 15°. This might contribute to explain high chorus intensities measured by CLUSTER on the dayside at high latitudes.

An improved forecast system for relativistic electrons at geosynchronous orbit

Here we provide a review of existing forecast models for Earth's outer radiation belt electrons, discuss some recent improvements to two of these models, and present a new and improved forecast system for relativistic electrons at GEO. For the first time, we can forecast at a local hour resolution around GEO using a statistical tool included in the system. This forecast system also includes several real‐time forecast models, two previously existing and one that is a new development. This new model incorporates an internal electron source, simulating local acceleration by wave‐particle interactions, and it proves to be the most accurate of the models in the system. For 2007–2008, it achieves +1 and +2 day prediction efficiencies of 0.90 and 0.63. We conclude this work with a discussion of how these models are currently operational and providing results to the community online in real time, and we also speculate on future possibilities to allow for forecasts with extended lead times and ranges throughout the rest of the outer radiation belt.

Research Spotlight: Wave‐particle interactions important in radiation belt dynamics

In the Earth's outer radiation belt the flux of energetic electrons and ions can vary significantly over short time periods in response to solar activity. These energetic particles can damage satellites, making it important to be able to understand and predict variability in the radiation belts. In recent years, scientists' understanding of how electrons are accelerated in the radiation belts has changed. Thorne reviews a variety of recent advances in understanding and modeling radiation belt dynamics, highlighting the importance of interactions of energetic particles with magnetosphere waves.

Radiation belt dynamics: The importance of wave‐particle interactions

The flux of energetic electrons in the Earth's outer radiation belt can vary by several orders of magnitude over time scales less than a day, in response to changes in properties of the solar wind instigated by solar activity. Variability in the radiation belts is due to an imbalance between the dominant source and loss processes, caused by a violation of one or more of the adiabatic invariants. For radiation belt electrons, non‐adiabatic behavior is primarily associated with energy and momentum transfer during interactions with various magnetospheric waves. A review is presented here of recent advances in both our understanding and global modeling of such wave‐particle interactions, which have led to a paradigm shift in our understanding of electron acceleration in the radiation belts; internal local acceleration, rather than radial diffusion now appears to be the dominant acceleration process during the recovery phase of magnetic storms.

Non‐stormtime injection of energetic particles into the slot‐region between Earth's inner and outer electron radiation belts as observed by STSAT‐1 and NOAA‐POES

The slot‐region between Earth's inner and outer electron radiation belts was observed on 24 February 2004 by the satellite STSAT‐1 to be populated by quasi‐trapped electrons of energy 100‐400 keV. This injection lasted for several hours and took place during a non‐stormtime substorm. This appears to be the first observation of a slot‐region electron injection that did not occur during a geomagnetic storm. We also report multi‐instrument observations of this event from NOAA‐POES and CPMN magnetic observatories, and we consider physical mechanisms that may account for the phenomenon.

Rapid scattering of radiation belt electrons by storm‐time EMIC waves

A storm main phase can produce a rapid depletion of electron fluxes in the Earth's outer radiation belt and the pitch‐angle scattering by the electromagnetic ion cyclotron (EMIC) waves is one mechanism that might account for the electron losses. To efficiently scatter the bulk of the electron population, below 1–2 MeV, the EMIC waves would need to have significant power very near a heavy ion gyrofrequency. We present a wave event at the storm main phase and carefully examine the wave spectrum to identify the energy range of electrons scattered by the waves. The EMIC waves exhibit power right below the He+ gyrofrequency and we estimate that they can interact with electrons having energies as low as 400 keV producing rapid scattering at almost all pitch‐angle values on the time scales of seconds. Our statistical analysis suggests that this event is not an exception; the majority of EMIC waves can scatter electrons with energies under 2 MeV. Our results show that EMIC waves can be one of the dominant radiation belt loss mechanisms during the storm main phase.

If the Sun is so quiet, why is the Earth ringing? A comparison of two solar minimum intervals

Observations from the recent Whole Heliosphere Interval (WHI) solar minimum campaign are compared to last cycle's Whole Sun Month (WSM) to demonstrate that sunspot numbers, while providing a good measure of solar activity, do not provide sufficient information to gauge solar and heliospheric magnetic complexity and its effect at the Earth. The present solar minimum is exceptionally quiet, with sunspot numbers at their lowest in 75 years and solar wind magnetic field strength lower than ever observed. Despite, or perhaps because of, a global weakness in the heliospheric magnetic field, large near‐equatorial coronal holes lingered even as the sunspots disappeared. Consequently, for the months surrounding the WHI campaign, strong, long, and recurring high‐speed streams in the solar wind intercepted the Earth in contrast to the weaker and more sporadic streams that occurred around the time of last cycle's WSM campaign. In response, geospace and upper atmospheric parameters continued to ring with the periodicities of the solar wind in a manner that was absent last cycle minimum, and the flux of relativistic electrons in the Earth's outer radiation belt was elevated to levels more than three times higher in WHI than in WSM. Such behavior could not have been predicted using sunspot numbers alone, indicating the importance of considering variation within and between solar minima in analyzing and predicting space weather responses at the Earth during solar quiet intervals, as well as in interpreting the Sun's past behavior as preserved in geological and historical records.

Storm‐dependent radiation belt electron dynamics

Using recently published electron phase space densities (PSD) as a function of L* (L* is approximately the radial distance in Earth radii at the equator) and time, energization and loss in the Earth's outer electron radiation belt were studied quantitatively and numerically using a radial diffusion model that included finite electron lifetimes and an internal source parameterized as a function of geomagnetic indices. We used PSD data at fixed values of the first and second adiabatic invariants, corresponding to electrons mirroring near the Earth's equator with an energy of ∼2.7 MeV at L* = 4. Model results for the second half of 2002 reproduced the average variations of the radiation belt electron PSD between L* = 2.5 and L* = 6 but with overprediction and underprediction at different times, implying that the same set of parameters cannot be applied to all storms. A detailed analysis of four individual storms showed that while electrons in three storms could be well simulated by energization from either radial diffusion only or internal heating only, incorporating both yielded the best results. For the other storm, an additional source of electrons was required to account for the enhanced PSD. The model results indicated that each storm is best simulated when a combination of radial diffusion and internal heating is used. Different storms required different magnitudes of radial diffusion and internal heating, and the relative contributions of these two acceleration mechanisms varied from storm to storm. A comparison of the results from different runs for the four storms and an analysis of the radial diffusion coefficients further suggest that internal heating contributes more to the enhancement of 2.7 MeV electrons at L* = 4 than radial diffusion.

Survey of ELF-VLF plasma waves in outer radiation belt observed by Cluster STAFF-SA experiment

Abstract. Various types of plasma waves have profound effects on acceleration and scattering of radiation belt particles. For the purposes of radiation belt modeling it is necessary to know statistical distributions of plasma wave parameters. This paper analyzes four years of plasma wave observations in the Earth's outer radiation belt obtained by the STAFF-SA experiment on board Cluster spacecraft. Statistical distributions of spectral density of different plasma waves observed in ELF-VLF range (chorus, plasmaspheric hiss, magnetosonic waves) are presented as a function of magnetospheric coordinates and geomagnetic activity indices. Comparison with other spacecraft studies supports some earlier conclusions about the distribution of chorus and hiss waves and helps to remove the long-term controversy regarding the distribution of equatorial magnetosonic waves. This study represents a step towards the development of multi-spacecraft database of plasma wave activity in radiation belts.

Radial transport in the outer radiation belt due to global magnetospheric compressions

Earth's outer radiation belt is populated by relativistic electrons that produce a complex dynamical response to varying geomagnetic activity. One fundamental process defining global state of the belt is radial transport of electrons across their drift shells. Radial transport is induced by resonant interaction of electron drift motion with ULF oscillations of electric and magnetic fields and is commonly believed to be a diffusive process. The goal of this paper is the analysis of radial transport due to typical ULF fluctuations in the inner magnetospheric fields. For this purpose a test-particle approach is used in the guiding center approximation. In particular we consider ULF oscillations due to global magnetospheric compressions. It is shown that typical pressure variations induce large-scale fluctuations in magnetic and inductive electric fields that produce a substantial impact on relativistic electrons. Electron motion becomes stochastic due to overlap of electron populations trapped in the vicinities of drift resonances with adjacent harmonics of the field spectrum. It is shown that in spite of the underlying stochasticity the radial diffusion limit is not fully attainable in the outer radiation belt. This is attributed to the fact that phase correlations in electron motion do not have time to decay due to finite size of the system. As a result collective motion of the outer belt electrons can exhibit large deviations from radial diffusion. Solution of the full Liouville's equation is required for accurate description of radial transport in the belt.

Review of modeling of losses and sources of relativistic electrons in the outer radiation belt I: Radial transport

In this paper, we focus on the modeling of radial transport in the Earth's outer radiation belt. A historical overview of the first observations of the radiation belts is presented, followed by a brief description of radial diffusion. We describe how resonant interactions with poloidal and toroidal components of the ULF waves can change the electron's energy and provide radial displacements. We also present radial diffusion and guiding center simulations that show the importance of radial transport in redistributing relativistic electron fluxes and also in accelerating and decelerating radiation belt electrons. We conclude by presenting guiding center simulations of the coupled particle tracing and magnetohydrodynamic (MHD) codes and by discussing the origin of relativistic electrons at geosynchronous orbit. Local acceleration and losses and 3D simulations of the dynamics of the radiation belt fluxes are discussed in the companion paper [Shprits, Y.Y., Subbotin, D.A., Meredith, N.P., Elkington, S.R., 2008. Review of modeling of losses and sources of relativistic electrons in the outer radiation belt II: Local acceleration and loss. Journal of Atmospheric and Solar-Terrestrial Physics, this issue. doi:10.1016/j.jastp.2008.06.014].

Energetic electron distributions fitted with a relativistic kappa‐type function at geosynchronous orbit

In this study, we utilize a recently introduced relativistic kappa‐type (KT) distribution function to model the omnidirectional differential flux of energetic electrons observed by the SOPA instrument on board the 1989–046 and LANL‐01A satellites at geosynchronous orbit. We derive a useful correlation between the differential flux and the distribution of particles which can directly offer those best fitting parameters (e.g., the number density N, the thermal characteristic speed θ and the spectral index κ) strongly associated with evaluation of the electromagnetic wave instability. We adopt the assumption of a nearly isotropic pitch angle distribution (PAD) and the typical LMFIT function in the program IDL to perform a non‐linear least squared fitting, and find that the new KT distribution fits well with the observed data during different universal times both in the lower and higher energies. We also carry out the direct comparisons with the generalized Lorentzian (kappa) distribution and find that kappa distribution fits well with observational data at the relatively lower energies but display deviations at higher energies, typically above hundreds of keV. Furthermore, the fitting spectral index κ basically takes 4, 5 or 6 while the fitting parameters N and θ are quite different due to different differential fluxes of electrons at different universal times. These results, which are applied to the case of a nearly isotropic PAD, demonstrate that the particle flux satisfies the power law not only at the lower energies but also at the relativistic energies, and the new KT distribution may present valuable insights into the dynamical features in those space plasmas (e.g., the Earth's outer radiation belts and the inner Jovian magnetosphere) where highly energetic particles exist.

Flux enhancement of the outer radiation belt electrons after the arrival of stream interaction regions

The Earth's outer radiation belt electrons increase when the magnetosphere is surrounded by the high‐speed solar wind stream, while the southward interplanetary magnetic field (IMF) is also known as an important factor for the flux enhancement. In order to distinguish the two different kinds of solar wind parameter dependence statistically, we investigate the response of the outer belt to stream interaction regions (SIRs). A total of 179 SIR events are identified for the time period from 1994 to 2005. We classify the SIR events into two groups according to the so‐called “spring‐toward fall‐away” rule: IMF sector polarity after the stream interface is toward in spring or away in fall (group A) and vice versa (group B). According to the Russell‐McPherron effect, groups A and B have a significant negative and positive offset of the IMF Bz after the stream interface, respectively. Comparing groups A and B by superposing about the stream interface, only IMF Bz dependence can be obtained because the other solar wind parameters change in the same manner. As a result, the greatest flux enhancement is found in the highest‐speed streams with a southward offset of the IMF Bz, indicating that only the solar wind speed by itself is not a sufficient condition for the large flux enhancement. It is also found that the large flux enhancement tends to be associated with weak geomagnetic activities with minimum Dst of about −50 nT on average, implying that the existence of intense magnetic storms is not essential for the flux enhancement.