Interplanetary Magnetic Field Conditions Research Articles

Energization of planetary pickup ions in the solar system

We study the solar wind‐induced ion escape from planetary atmospheres at different radial heliospheric distances in the solar system. We derive histograms of the gyroaveraged E×B velocities, energies, and Larmor radii of planetary ions in the solar wind at Mercury, Venus, Earth, and Mars. The statistical analysis is based on the interplanetary Pioneer Venus Orbiter and OMNI solar wind data sets. In addition to the energization in the undisturbed solar wind we also model how planetary heavy ions get energized in the solar wind interaction of an unmagnetized planet at different distances to the Sun. We found that due to the Parker spiral, pickup ions are expected to be found on average at lower energies and at velocities more perpendicular to the solar wind flow, the closer to the Sun a planet or a comet is. According to a global hybrid simulation, planetary heavy ion energization is influenced qualitatively in a similar way in the presence of an induced magnetosphere than in the upstream solar wind under different Parker spiral angles due to fact that the structure of an induced magnetosphere depends strongly on the interplanetary magnetic field and solar wind conditions. Finally, the energization and dynamics of the pickup ions vary considerably with the solar activity. The variation is stronger the farther away from the Sun an object is. The Larmor radii of the pickup ions are largest during a solar minimum while the pickup ion energies are highest during the declining phase of a solar cycle.

Statistical study of foreshock cavitons

Abstract. In this work we perform a statistical analysis of 92 foreshock cavitons observed with the Cluster spacecraft 1 during the period 2001–2006. We analyze time intervals during which the spacecraft was located in the Earth's foreshock with durations longer than 10 min. Together these amount to ~ 50 days. The cavitons are transient structures in the Earth's foreshock. Their main signatures in the data include simultaneous depletions of the magnetic field intensity and plasma density, which are surrounded by a rim of enhanced values of these two quantities. Cavitons form due to nonlinear interaction of transverse and compressive ultra-low frequency (ULF) waves and are therefore always surrounded by intense compressive ULF fluctuations. They are carried by the solar wind towards the bow shock. This work represents the first systematic study of a large sample of foreshock cavitons. We find that cavitons appear for a wide range of solar wind and interplanetary magnetic field conditions and are therefore a common feature upstream of Earth's quasi-parallel bow shock with an average occurrence rate of ~ 2 events per day. We also discuss their observational properties in the context of other known upstream phenomena and show that the cavitons are a distinct structure in the foreshock.

Relativistic electron fluxes dropout in the outer radiation belt under different solar wind conditions

In this study, we apply superposed epoch analysis to the 1.5–6.0 MeV electron flux dropout events observed on Solar, Anomalous, Magnetospheric Particle Explorer satellite for 110 magnetic storms related to coronal mass ejections (CMEs) associated with interplanetary shocks during 1998–2003, which can help to study one of the scientific objectives of the recently launched Van Allen Probe—to determine and quantify the mechanisms of the losses in the outer radiation belt. Results obtained in this paper show that the impact of high solar wind dynamic pressure (Pdy) on the magnetosphere would lead to much larger electron flux dropout than low dynamic pressure. Furthermore, it is shown that southward interplanetary magnetic field (IMF) condition can result in more significant dropout compared with northward IMF condition. In addition, the largest local dropout is caused by high Pdy with northward IMF at L∼5.1. Dropouts under high Pdy and southward IMF conditions are the largest, whereas dropouts under low Pdy and northward IMF conditions are the smallest. Our study tackles the problem of quantifying the dropouts of electrons by calculating the radiation belt content index and finding the spatial distribution of dropout and the location of maximum dropout. Another finding is that Pdy and IMF affect the dropouts in CME‐driven storms. These new findings provide insight into which mechanisms play a more important role in different dropout events.

Why have geomagnetic storms been so weak during the recent solar minimum and the rising phase of cycle 24?

The minimum following solar cycle 23 was the deepest and longest since the dawn of the space age. In this paper we examine geomagnetic activity using Dst and AE indices, interplanetary magnetic field (IMF) and plasma conditions, and the properties and occurrence rate of interplanetary coronal mass ejections (ICMEs) during two periods around the last two solar minima and rising phases (Period 1: 1995–1999 and Period 2: 2006–2012). The data is obtained from the 1-h OMNI database. Geomagnetic activity was considerably weaker during Period 2 than during Period 1, in particular in terms of Dst. We show that the responses of AE and Dst depend on whether it is solar wind speed or the southward IMF component (BS) that controls the variations in solar wind driving electric field (EY). We conclude that weak Dst activity during Period 2 was primarily a consequence of weak BS and presumably further weakened due to low solar wind densities. In contrast, solar wind speed did not show significant differences between our two study periods and the high-speed solar wind during Period 2 maintained AE activity despite of weak BS. The weakness of BS during Period 2 was attributed in particular to the lack of strong and long-duration ICMEs. We show that for our study periods there was a clear annual north–south IMF asymmetry, which affected in particular the intense Dst activity. This implies that the annual amount of intense Dst activity may rather be determined by the coincidence of what magnetic structure the strong ICMEs encountering the Earth have than by the solar cycle size.

Earth's collision with a solar filament on 21 January 2005: Overview

AbstractOn 21 January 2005, one of the fastest interplanetary coronal mass ejections (ICME) of solar cycle 23, containing exceptionally dense plasma directly behind the sheath, hit the magnetosphere. We show from charge‐state analysis that this material was a piece of the erupting solar filament and further, based on comparisons to the simulation of a fast CME, that the unusual location of the filament material was a consequence of three processes. As the ICME decelerated, the momentum of the dense filament material caused it to push through the flux rope toward the nose. Diverging nonradial flows in front of the filament moved magnetic flux to the sides of the ICME. At the same time, reconnection between the leading edge of the ICME and the sheath magnetic fields worked to peel away the outer layers of the flux rope creating a remnant flux rope and a trailing region of newly opened magnetic field lines. These processes combined to move the filament material into direct contact with the ICME sheath region. Within 1 h after impact and under northward interplanetary magnetic field (IMF) conditions, a cold dense plasma sheet formed within the magnetosphere from the filament material. Dense plasma sheet material continued to move through the magnetosphere for more than 6 h as the filament passed by the Earth. Densities were high enough to produce strong diamagnetic stretching of the magnetotail despite the northward IMF conditions and low levels of magnetic activity. The disruptions from the filament collision are linked to an array of unusual features throughout the magnetosphere, ionosphere, and atmosphere. These results raise questions about whether rare collisions with solar filaments may, under the right conditions, be a factor in producing even more extreme events.

Kelvin‐Helmholtz instability of the magnetospheric boundary in a three‐dimensional global MHD simulation during northward IMF conditions

We present results of a global, fully three‐dimensional, high‐resolution magnetohydrodynamic (MHD) simulation of the magnetosphere during steady northward interplanetary magnetic field (IMF) conditions. We investigate the stability of the magnetospheric boundary with respect to the growth of the Kelvin‐Helmholtz instability (KHI) driven by the velocity shear between the nearly stagnant magnetospheric plasma and the magnetosheath flow past it. We find the magnetospheric boundary to be globally unstable, including the high‐latitude boundary layer (meridional plane), where magnetic tension is not sufficient to stabilize the growth of oscillations. Roughly beyond the terminator, global modes coupled into the surface modes become most apparent, so that the entire body of the magnetosphere is engaged in an oscillatory motion. The wave vector of the surface oscillations has a component perpendicular to the background flow and tangential to the shear layer (in the equatorial plane, kz component of the wave vector), which is consistent with the generation of field‐aligned currents that flow on closed field lines between the inner portion of the boundary layer and the ionosphere. The distribution of wave power in the equatorial plane is consistent with the existence of a double‐vortex sheet, with vortex trains propagating along the inner and outer edges of the boundary layer. The double‐vortex sheet is most apparent in the simulation past the terminator plane but is transient and appears to be unstable and is most likely a consequence of nonlinear development of the velocity shear layer with a finite width. For the simulation with the solar wind velocity of 600 km/s, we find the width of the layer to be Δ≈1 RE at the terminator and the phase speed there to be similar to half of the total velocity drop across the layer (∼440 km/s), which is expected for a shear layer with uniform background density. We calculate the spatial growth rate for the dominant frequency mode in this region (∼4.4 mHz) to be ∼0.19RE−1, which is in excellent agreement with linear theory. For this mode, we find kΔ≈0.9, where k is the wave number, which corresponds to the fastest growing mode predicted by the linear theory. Finally, we find that the plasma compressibility is a key factor in controlling the growth rate of the KHI at the magnetosphere flanks in our simulation.

Magnetic interconnection of Saturn's polar regions: comparison of modelling results with Hubble Space Telescope UV auroral images

Abstract. We consider the magnetic interconnection of Saturn's northern and southern polar regions controlled by the interplanetary magnetic field (IMF), studying in particular the more complex and interesting case of southward IMF, when the Kronian magnetospheric magnetic field structure is the most twisted. The simpler case of northward IMF is also discussed. Knowledge of the magnetospheric magnetic field structure is very significant, for example, for investigation of the electric fields and field-aligned currents in Saturn's environment, particularly those which cause the auroral emissions. Here we modify the paraboloid magnetospheric magnetic field model employed in previous related studies by including higher multipole terms in Saturn's internal magnetic field, required for more detailed considerations of inter-hemispheric conjugacy, together with inclusion of a spheroidal boundary at the ionospheric level. The model is employed to map Southern Hemisphere auroral regions observed by the Hubble Space Telescope (HST) in 2008 under known IMF conditions to both the equatorial plane and the northern ionosphere. It is shown that the brightest auroral features map typically to the equatorial region between the central ring current and the outer magnetosphere, and that auroral features should be largely symmetric between the two hemispheres, except for a small poleward displacement and latitudinal narrowing in the Northern Hemisphere compared with the Southern Hemisphere due to the quadrupole field asymmetry. The latter features are in agreement with the conjugate auroras observed under near-equinoctial conditions in early 2009, when IMF data are not available.

IMF-induced escape of molecular ions from the Martian ionosphere

Abstract. Since Mars does not possess a significant global intrinsic magnetic field, the solar wind interacts directly with the Martian ionosphere and can induce ion escapes from it. Phobos-2 and recent Mars Express (MEX) observations have shown that the escaping ions are O+ as well as molecular O2+ and CO2+. While O+ escape can be understood by the ion pick-up of non-thermal O corona extended around the planet, regarding the heavy molecular O2+ and CO2+, which are buried in the lower ionosphere, a novel escape mechanism needs to considered. Here we attack this problem by global magnetohydrodynamic (MHD) simulations. First, we clarify the global structure of the streamlines that result from the interaction with the solar wind. Then, by focusing on the streamlines that dip into the low-altitude part of the dayside ionosphere, we investigate the escape path of the molecular ions. The effects of the interplanetary magnetic field (IMF) on the molecular ion escape process are investigated by comparing the results with and without IMF. IMF has little effect on O+ escape via ion pick-up mediated by solar wind electron impact ionization of the O corona. O2+ and CO2+ are shoveled from the low-altitude regions of the dayside ionosphere by magnetic tension in the presence of IMF. These ions are pulled by the U-shaped field lines to the north and south poles, and at the terminator, they are concentrated in the noon–midnight meridian plane. These ions remain confined to the noon–midnight plane as they are transported to the nightside to form the tail ray. Then they escape along the streamlines open to the interplanetary space. Under a typical solar wind and IMF condition expected at Mars, O+, O2+ and CO2+ escape fluxes are 8.0 × 1023, 3.5 × 1023 and 5.0 × 1022 ion s−1, respectively, which are in good agreement with the MEX observations.

Global features of Kelvin‐Helmholtz waves at the magnetopause for northward interplanetary magnetic field

We investigate the global features of Kelvin‐Helmholtz waves (KHW) at the low‐latitude magnetopause for constant northward interplanetary magnetic field conditions, using global magnetohydrodynamic simulations. The root‐integrated power (RIP) of X component of bulk velocity is employed to analyze the magnetopause mode of KHW along the boundary layer. The RIP distribution of the outer KHW is much broader than that of the inner one, and the maximum amplitude of global KHW occurs near the dawn/dusk terminator regions. In the dayside magnetopause, the phase of the waveform at middle latitudes leads to that at low latitudes, while the situation reversed in the nightside. The global evolution of KHW phases is a representation of an interesting feature that the axis of the Kelvin‐Helmholtz vortex aligns with the geomagnetic field lines. We suggest that the reported features may also exist in other KHW active regions with flow‐sheared plasma.

Investigation of the interhemispheric asymmetry in reverse convection near solstice during northward interplanetary magnetic field conditions using MHD simulations

AbstractThe reverse convection potential under northward interplanetary magnetic field (IMF) is significantly larger in the summer ionosphere than in the winter. In this study, we use the Coupled Magnetosphere‐Ionosphere‐Thermosphere model to simulate a Northern Hemisphere winter event where observations have shown asymmetry in both the reverse convection strength between hemispheres and the magnetic field topology associated with the ionospheric reverse convection cells. We show that a topological asymmetry, in which reconnection between the geomagnetic field and the IMF occurs in the summer hemisphere, first drives the interhemispheric asymmetry in reverse convection strength rather than an interhemispheric asymmetry in ionospheric conductivity. We find a large amount of overdraped open magnetic flux connected to the summer hemisphere that results from this reconnection bypasses the winter hemisphere x line and reconnects with the IMF again in the summer hemisphere lobe. This leads to a large amount of circulating lobe flux in the summer hemisphere and stagnant lobe flux in the winter hemisphere, while maintaining a divergence‐free field, as well as the weaker reverse convection potential in the winter hemisphere ionosphere.

Localized dayside proton aurora at high latitudes

Two types of dayside localized proton‐induced auroral emissions on the dayside were identified in SI12 images from the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft. “Auroral oval spots” occurred at or toward the poleward edge of the dayside proton auroral oval. “Polar cap spots” occurred well into the polar cap. The polar cap spots occurred when the interplanetary magnetic field (IMF) was strongly northward and radially aligned along the Sun‐Earth line, which results in reconnection with northern lobe field lines. For the auroral oval spots that occurred during northward‐IMF only, there was a large azimuthal component of the IMF. During northward IMF conditions, a large azimuthal component of the IMF leads to reconnection at high latitudes on closed geomagnetic field lines. The reconnection site would map closer to auroral oval latitudes, which is consistent with the location of the auroral oval spots. The auroral oval spots were found to be much larger in size and total intensity than the polar cap spots, but the average intensities of the two types of spots were comparable. The average intensity of the auroral oval spots was proportional to the solar wind dynamic pressure and solar wind density, suggesting that the average intensity of the auroral oval spots is dependent on the number of protons precipitating from the reconnection site, rather than the proton energy. No clear trend was found for the polar cap spots, but there was a notable lack of polar cap spots for high solar wind dynamic pressures and densities.

Correlation of scintillation occurrence with interplanetary magnetic field reversals and impact on Global Navigation Satellite System receiver tracking performance

Ionospheric scintillation is characterized by rapid fluctuations in the amplitude and phase of transionospheric radio signals as they pass through small scale ionospheric plasma density irregularities. Over the auroral regions, the occurrence of scintillation is associated with large‐scale plasma structures and is mainly enhanced during geomagnetic storms. Scintillation can cause cycle slips and degrade the positioning accuracy in Global Navigation Satellite System (GNSS) receivers. This paper investigates the correlation between the southward reversals of the interplanetary magnetic field (IMF) Bz and the occurrence of scintillation, along with the consequential impact on the tracking performance of a GNSS receiver located at a high‐latitude station, Bronnoysund (geographic latitude 65°N) in Norway. The analysis revealed that the occurrence of scintillation at this station is largely controlled by the IMF conditions. The receiver tracking performance under strong scintillations is evaluated by the receiver PLL (phase‐locked loop) tracking jitter variance. The dependence of the PLL jitter variance on phase scintillation can be best represented by a quadratic fit.

Long‐term analysis of ionospheric polar patches based on CHAMP TEC data

Total electron content (TEC) from LEO satellites offers great possibility to sound the upper ionosphere and plasmasphere. This paper describes a method to derive absolute TEC observations aboard CHAMP considering multipath effects and receiver differential code bias. The long‒term data set of 9 years GPS observations is used to investigate the climatological behavior of high‒latitude plasma patches in both hemispheres. The occurrence of polar patches has a clear correlation with the solar cycle, which is less pronounced in the Southern Hemisphere (SH). Summed over all years, we observed a higher number of patches in the SH. The maximum occurrence rate of patches has been found at the dayside polar cusp during 12:00–18:00 MLT (magnetic local time) supporting the mechanisms for patch creation by local particle precipitation and by intrusion of subauroral plasma into the polar cap through tongues of ionization (TOIs). The latter mechanism seems to be even more important in the SH. Investigating the patches in comparison with interplanetary magnetic field (IMF) conditions, we found that decreased IMF Bz and enhanced merging electric field preceded the patch observation; hence, patch creation follows a period of enhanced solar wind input into the magnetosphere/ionosphere. We further found an annual cycle in patch occurrence with maxima at equinox and December solstice and a June solstice minimum which reflects the global ionospheric seasonal asymmetry in electron density. We suggest that enhanced TEC at midlatitudes and low latitudes during December solstice provides a greater possibility to transport high‐density plasma to the polar region through the buildup of TOIs.

Direct observations of the role of convection electric field in the formation of a polar tongue of ionization from storm enhanced density

AbstractWe examine the relationship of convection electric fields to the formation of a polar cap tongue of ionization (TOI) from midlatitude plumes of storm enhanced density (SED). Observations from the geomagnetic storm on 26–27 September 2011 are presented for two distinct SED events. During an hour‐long period of geomagnetic activity driven by a coronal mass ejection, a channel of high‐density F region plasma was transported from the dayside subauroral ionosphere and into the polar cap by enhanced convection electric fields extending to middle latitudes. This TOI feature was associated with enhanced HF backscatter, indicating that it was the seat of active formation of small‐scale irregularities. After the solar wind interplanetary magnetic field conditions quieted and the dayside convection electric fields retreated to higher latitudes, an SED plume was observed extending to, but not entering, the dayside cusp region. This prominent feature in the distribution of total electron content (TEC) persisted for several hours and elongated in magnetic local time with the rotation of the Earth. No ionospheric scatter from SuperDARN radars was observed within this SED region. The source mechanism (enhanced electric fields) previously drawing the plasma from midlatitudes and into the polar cap as a TOI was no longer active, resulting in a fossil feature. We thus demonstrate the controlling role exercised by the convection electric field in generating a TOI from midlatitude SED.

Dayside poleward moving auroral forms and ionospheric convection under stable interplanetary magnetic field (IMF) conditions

Using high temporal resolution optical data obtained from three-wavelength all-sky imagers over six winters continuously at Yellow River Station (78.92°N, 11.93°E) in Arctic, we statistically investigated the dependence of location of poleward moving auroral forms (PMAFs) on the interplanetary magnetic field (IMF) Bz and By components as a function of MLT and MLAT under stable IMF conditions. It is found that more PMAFs occurred in lower latitude for Bz 0. We found that the PMAFs were distributed over a wide range of MLT when Bz 0 and postnoon for IMF By < 0 associating with the effect of the high-latitude reconnection, which is largely consistent with the theoretical model of the convection flow.

Chang’E-1 observations of pickup ions near the Moon under different interplanetary magnetic field conditions

Detailed features of the near-Moon pickup ions under different interplanetary magnetic field (IMF) conditions are investigated using data obtained from the solar wind ion detector (SWID-B) onboard Chang'E-1. In the event studied, Chang'E-1 was on a noon-midnight meridian orbit and the field-of-view of the SWID-B was in the satellite orbital plane. The observations show that the pickup energy detected depends not only on where these particles are detected but also on their incident angles. As the spacecraft moved into the wake from the South Pole along the midnight meridian, wider incident angle distributions were measured. When IMF Bx was significant, the pickup ions had a strong velocity component parallel to the magnetic field, and the efficiency of acceleration was reduced when the IMF By decreased. The back tracking calculations show that the possible source of the pickup ions is solar wind ions scattered/reflected on the lunar surface in a wide area over the dayside of the Moon, from both magnetic and nonmagnetic anomaly regions. A three dimensional analysis performed on the asymmetric properties of the pick-up ions can explain most of the particle behavior observed near the Moon under different IMF conditions.

Dynamics of the foreshock compressional boundary and its connection to foreshock cavities

AbstractWe use several global hybrid (kinetic ions, fluid electrons) simulation runs for steady and time‐varying interplanetary magnetic field (IMF) conditions to examine the dynamics of the foreshock compressional boundary (FCB) and its connection to foreshock cavities. The results demonstrate that for steady IMF conditions, the FCB forms and evolves over a long period of time due to the dynamics of the bow shock and ion foreshock. Formation of the FCB is tied to the generation and nonlinear evolution of ULF waves associated with large‐amplitude fluctuations in magnetic field and density within the foreshock. As a result, even during steady IMF conditions, the transitions in the magnetic field strength and direction across an FCB evolve. Although the FCB itself is associated with increases in the magnetic field strength and density, these quantities are reduced on the turbulent side of the FCB as compared to the pristine solar wind. Hybrid simulations with time‐varying IMF have been performed to examine the relationship between the FCB and foreshock cavities generated under two possible scenarios. In the first scenario, a bundle of field lines connects to an otherwise quasi‐perpendicular bow shock and results in the formation of a finite‐sized foreshock region that travels with this bundle of field lines as it connects to different parts of the bow shock surface. Two FCBs bound the traveling foreshock region. In the second scenario, solar wind discontinuities cause the IMF cone angle (angle between the IMF and the solar wind flow direction) to vary and thereby modify the foreshock geometry and the position of the FCB. We demonstrate that structures similar to foreshock cavities bounded by FCBs form in both scenarios. We show that the two scenarios cannot be distinguished based on convecting or nonconvecting FCBs. We also demonstrate that depending on spacecraft location and the nature of the solar wind discontinuities, foreshock cavities may be bounded by an FCB on one side and a foreshock bubble on the other.

Magnetospheric Boundary Layer Structure and Dynamics as Seen From Cluster and Double Star Measurements

In this review, we discuss the structure and dynamics of the magnetospheric Low-Latitude Boundary Layer (LLBL) based on recent results from multi-satellite missions Cluster and Double Star. This boundary layer, adjacent to the magnetopause on the magnetospheric side, usually consists of a mixture of plasma of magnetospheric and magnetosheath origins, and plays an important role in the transfer of mass and energy from the solar wind into the magnetosphere and subsequent magnetospheric dynamics. During southward Interplanetary Magnetic Field (IMF) conditions, this boundary layer is generally considered to be formed as a result of the reconnection process between the IMF and magnetospheric magnetic field lines at the dayside magnetopause, and the structure and plasma properties inside the LLBL can be understood in terms of the time history since the reconnection process. During northward IMF conditions, the LLBL is usually thicker, and has more complex structure and topology. Recent observations confirm that the LLBL observed at the dayside can be formed by single lobe reconnection, dual lobe reconnection, or by sequential dual lobe reconnection, as well as partially by localized cross-field diffusion. The LLBL magnetic topology and plasma signatures inside the different sub-layers formed by these processes are discussed in this review. The role of the Kelvin-Helmholtz instability in the formation of the LLBL at the flank magnetopause is also discussed. Overall, we conclude that the LLBL observed at the flanks can be formed by the combination of processes, (dual) lobe reconnection and plasma mixing due to non-linear Kelvin-Helmholtz waves.

Multi-Periodic Auroral and Thermospheric Variations in 2006

A data survey reveals multiple periodic variations in auroral hemispheric power (HP) and thermospheric composition (O/N2) in 2006. The periods include 27, 13 - 14, 9, and 6.7 days. These periods of 13 - 14, 9 and 6.7 days are essentially the harmonics of the 27-day solar rotation. Similar multi-periodicities were found in the dayside magnetic merging rate (MMR) (Newell et al. 2007) which depends upon solar wind speed (V), magnitude (BT) and clock angle (θc) of interplanetary magnetic field (IMF). A high correlation coefficient (0.93) between MMR and HP indicates MMR is the driver of the periodic variations. While high solar wind speed associated with coronal holes plays an important role in the HP variations, IMF BT is equally important. The term [BT 2/3 sin 8/3 (θc /2)] is even more important as its correlation coefficient with HP is higher than that for BT or solar wind speed. Nevertheless, MMR has the highest correlation with HP. Similar results were seen in the 2005 data where the 9-day variation is dominant. These results indicate that both solar wind speed and IMF conditions are required for accurate specification of periodic variations in aurora hemispheric power and thermosphere composition. Key word: High speed solar wind, Aurora, Thermosphere, Periodic variation

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.