Effect of the ring current on preconditioning the magnetosphere for steady magnetospheric convection

Authors


Abstract

[1] Steady magnetospheric convection (SMC) occurs when reconnection rates on the dayside and in the distant tail are balanced. An enhanced ring current could support this, by stabilizing the tail against near-Earth reconnection and by facilitating the prompt return of magnetic flux to the dayside. We use 32 years of magnetic index and solar wind data to study the effect of the ring current on preconditioning Earth's magnetosphere for SMC. The ring current is found to be enhanced during SMCs. Solar wind driving is similar but the ring current is weaker before the SMC, and the ring current is similar but solar wind driving is either weaker or stronger after the SMC than during it. This indicates that the magnetosphere cannot enter the SMC mode until the ring current has enhanced sufficiently for the prevailing driving conditions, and that when the driving exceeds a certain level, the ring current stabilization will fail.

1 Introduction

[2] When the interplanetary magnetic field (IMF) points southward, it can reconnect with Earth's northward-pointing magnetic field at the dayside magnetopause. The resulting field lines are anchored to either the northern or southern polar cap at one end, while the other end is dragged tailward by the solar wind. In the magnetotail, the oppositely directed field lines from the northern and southern hemispheres encounter again and reconnect. One of the resulting field lines is connected to both polar caps and migrates back toward the dayside, while the other is disconnected from the Earth and escapes downstream. This process is known as convection [Dungey, 1961].

[3] A common mode of response of the magnetosphere to enhanced solar wind energy input is a cycle of loading and unloading of magnetic flux (e.g., substorm [Caan et al., 1978]). When the dayside and nightside reconnection rates are occasionally balanced, a mode called steady magnetospheric convection (SMC) results [Pytte et al., 1978].

[4] An SMC event is an extended period of enhanced convection uninterrupted by bursty loading-unloading cycles. The extended period generally means several substorm durations or recurrence times such that the absence of substorms can be verified [e.g., Sergeev et al., 1996]. Enhanced convection, on the other hand, generally requires a southward IMF. However, there is no need for the solar wind conditions to remain completely unchanged for the entire SMC duration. It should be sufficient that the flux opened at the dayside magnetopause can be closed and returned to the dayside without too much delay. Indeed, it has been shown that both IMF direction and amplitude can vary during an SMC [O'Brien et al., 2002].

[5] During loading, magnetic flux opened by dayside reconnection accumulates in the tail lobes, leading to stretching of the tail magnetic field and intensification and thinning of the cross-tail current sheet [McPherron et al., 1973; McPherron, 1979]. At the onset of unloading, a new tail X-line forms near the Earth where the current sheet has thinned, on average between x=−16 R E and −20 R E. When the fast earthward plasma sheet flows produced by the tail reconnection enter the transition region between the tail-like and dipolar field lines, the flows start to brake and the magnetic flux carried by them piles up. This large-scale dipolarization of the tail magnetic field begins between x=−7 R E and −10 R E and expands tailward, azimuthally, and earthward [e.g., Miyashita et al., 2009]. As a consequence, the cross-tail current reduces and the substorm current wedge [McPherron et al., 1973] forms [e.g., Shiokawa et al., 1998]. The ionospheric closure of the wedge causes a sharp decrease in the magnetic field observed on ground in the auroral region. Toward the end of the unloading, the substorm current wedge decays and the tail returns to its ground state with a moderately stretched magnetic field configuration. The near-Earth X-line retreats tailward, forming a new distant X-line.

[6] During an SMC, on the other hand, reconnection is thought to occur quasi-steadily in the distant tail without the formation of a new near-Earth X-line [Pytte et al., 1978]. This is supported by the relative abundance of earthward and tailward plasma sheet flows observed during continuous magnetospheric dissipation (a mode that can be considered similar to SMC) [Tanskanen et al., 2005].

[7] O'Brien et al. [2002] reported that during SMCs, the solar wind velocity is typically below 450 km/s and IMF Bz≈−3 nT. However, they also demonstrated that the occurrence of such solar wind conditions alone is not sufficient for driving an SMC. In their example, similar solar wind conditions produced an SMC in one case and a substorm in another. The main difference between the two events was the preconditioning of the magnetosphere: The SMC was preceded by a period of modest activity, while the substorm was preceded by very quiet conditions. Adjustment of the location or intensity of nightside reconnection was suggested as the simplest explanation.

[8] Milan [2009] speculated that the intensity of the ring current plays a role in controlling the rate at which open flux is reclosed in the magnetotail, by modifying the level of stretching of the tail field and hence its stability to the onset of nightside reconnection. The active conditions observed by O'Brien et al. [2002] to precede the example SMC period could have led to intensification of the ring current whereas the quiet period preceding the substorm suggests a quiet ring current. Most SMCs are preceded by substorm activity [e.g., DeJong et al., 2009; Kissinger et al., 2012a].

[9] Kissinger et al. [2012b] suggested that balancing of the dayside and nightside reconnection rates is not a sufficient condition for an SMC state, but that the magnetic flux closed by the nightside reconnection needs to be promptly returned to the dayside to replace the flux depleted by the dayside reconnection. They showed that the average total pressure in the inner magnetosphere is higher during SMCs than quiet intervals or isolated substorms. Thus, during isolated substorms, fast earthward flows tend to be directed toward the inner nightside magnetosphere, where magnetic flux then piles up. During SMCs, on the other hand, the enhanced pressure deflects the fast flows toward the dawn and dusk flanks, facilitating the prompt transportation of magnetic flux toward the dayside.

[10] Thus, both continuous nightside reconnection and efficient transport of the closed magnetic flux to the dayside seem to be essential for the magnetosphere to reach and maintain an SMC state. An enhanced ring current could both stabilize the tail against the formation of a new reconnection site close to Earth and contribute to the increase of the total pressure in the inner magnetosphere. In this study, we will examine the possible connection between the SMC state and the ring current.

2 Observations

[11] We have used the auroral electrojet index (AL) [Davis and Sugiura, 1966] to identify SMC intervals. The ring current intensity was estimated from the azimuthally symmetric part of the mid-latitude geomagnetic index in the horizontal dipole pole direction (SYM-H) [Iyemori and Rao, 1996]. The AL and SYM-H indices at 1 min resolution were obtained from the World Data Center for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/index.html). Solar wind data at 1 h resolution propagated to the Earth's bow shock nose were extracted from NASA/GSFC's OMNI data set through the OMNIWeb interface (http://omniweb.gsfc.nasa.gov/).

[12] We defined an SMC as an (1) at least 90 min long interval [O'Brien et al., 2002; Kissinger et al., 2012b] during which (2) AL≤−130 nT and (3) AL(t)−AL(t−1 min)≥−25 nT [O'Brien et al., 2002; Partamies et al., 2009a, 2009b]. Condition (2) was intended for finding intervals of enhanced convection and condition (3) for ruling out any sharp drops in AL, which are generally associated with substorm onsets. For condition (2), we chose AL≤−130 nT instead of the more commonly used AE≥200 nT [Sergeev et al., 1996; O'Brien et al., 2002; Partamies et al., 2009a, 2009b], because AL is less sensitive to seasonal conductivity changes than AE [McWilliams et al., 2008]. These criteria yielded 3492 SMCs for the years 1981–2012 with a minimum, maximum, and mean durations of 90 min, 539 min (8 h 59 min), and 124 min (2 h 4 min), respectively. About 2% (70) of the SMCs lasted for at least 4 h, and about 0.2% (6) for at least 6  h.

[13] Figures 2 and 1 show the percentual duration of SMC observations relative to all observations as a function of the SYM-H index and the z-component of IMF (Bz, Figure 2) and the x-component of the solar wind velocity (Vx, Figure 1). For the solar wind data, we have used the geocentric solar magnetospheric (GSM) coordinates. SYM-H is used as a proxy for the intensity of the ring current. The percentages were calculated by dividing the total SMC time for each SYM-H and IMF Bz or solar wind Vx bin by the total observation time for that bin. The 31 SYM-H, 25 Bz, and 24 Vx bins resulted in 775 bins in Figure 2 and 744 bins in Figure 1. If the 32 years of data were evenly distributed over these bins, there would be approximately 15 days of data in each SYM-H- Bz bin and 16 days of data in each SYM-H- Vxbin. Or, considering missing data points and points outside of the included ranges, 11 and 10 days in the Bz and Vx plots, respectively. The gray curves in Figures 2 and 1 mark the regions inside of which the integrated observation time exceeds this even distribution limit, i.e., where the majority of the data points are concentrated. The magenta curve shows the same for the SMC data distribution.

Figure 1.

Same as Figure 2 except that the data are shown as a function solar wind Vx instead of IMF Bz. Above the visually placed white line, the occurrence frequency of SMCs appears to increase.

Figure 2.

Duration of SMC observations relative to all observations (%) as a function of the SYM-H index and IMF Bz. The gray and magenta curves mark the regions inside of which the majority of all data points and SMC data points, respectively, are concentrated.

[14] Figure 2 shows that the occurrence frequency of SMCs increases for moderately southward IMF Bzconditions. This is in agreement with both O'Brien et al. [2002] (IMF Bz∼−3 nT during SMCs) and Tanskanen et al. [2005] (IMF Bz>−5 nT during continuous magnetospheric dissipation events). Furthermore, slightly enhanced ring current (decreased SYM-H) conditions also increase the occurrence frequency of SMCs.

[15] Figure 1 shows that the occurrence frequency of SMCs clearly increases for slow solar wind speed conditions. This is in agreement with both O'Brien et al. [2002] (solar wind speed generally below 450 km/s during SMCs) and Partamies et al. [2009a, 2009b] (solar wind speed typically about 400 km/s during SMCs). However, it appears that SMCs can also occur for higher solar wind speeds if the ring current is sufficiently strong. As Vx decreases, the occurrence frequency of SMCs increases for larger SYM-H levels. The white line in Figure 1 marks the minimum Vx as a function of SYM-H above which the occurrence frequency of SMCs seems to increase. The approximate condition for the increased occurrence frequency of SMCs

display math(1)

was found by visual inspection of the figure.

3 Discussion

[16] We have shown that in addition to a southward IMF, an enhanced ring current appears to be one of the conditions that allow the magnetosphere to enter and maintain the SMC mode. Two possible mechanisms could be the following: (1) increase of the tail Bz, which stabilizes the tail against the onset of near-Earth reconnection [Milan, 2009] and (2) increase of the tail magnetic pressure, which causes the flows coming from farther down tail to deviate around the Earth at larger radial distances [Kissinger et al., 2012b]. In order to illustrate these effects, Figures 4 and 3 show the increase of Bz and Pmag=B2/2μ0, respectively, at the equatorial plane as a function of radial distance from the Earth as caused by a simplified model ring current. The model comprises a current loop located at the equatorial plane at the radial distance of 5R E. The increase is shown as a percentage of the corresponding dipole field value. The different curves correspond to different amplitudes of the current and, thus, different SYM-H values. We assume that SYM-H is attributed to only the ring current, although in reality the cross-tail current can contribute a few tens of percent [Ohtani et al., 2001]. Thus, for a given value of SYM-H, Bz and Pmag would be somewhat smaller than what is shown here.

Figure 3.

Same as Figure 4 except for the magnetic pressure.

Figure 4.

Increase of Bz at the equatorial plane as a function of radial distance from the Earth as caused by a current loop at 5 R E. The increase is shown as a percentage of the corresponding dipole field value. The different curves correspond to different amplitudes of the current and, thus, different SYM-H values.

[17] According to Miyashita et al. [2009], near-Earth X-line forms between x=−20 R E and −16 R E. At x=−20 R E, the dipole contribution to Bz is 4 nT and the ring current contribution (for the model SYM-H =−41 nT) 0.3 nT or 9%. At x=−16 R E, the corresponding numbers are 8 and 0.7 nT or 9%. Flux pile-up, on the other hand, begins between x=−10 R E and −7 R E. At x=−10 R E, the dipole contribution to Pmag is 0.4 nPa and the ring current contribution 0.005 nPa or 1%. At x=−7 R E, the corresponding numbers are 3 nPa and 0.1 nPa or 3%. According to Kissinger et al. [2012b], the increase in total pressure from quiet to SMC conditions is of the order of a few 0.1 nPa. Hence, the ring current contribution to total pressure is rather insignificant, and any effect in favor of the SMC mode would most likely be through the stabilizing effect of the Bz increase.

[18] In order to examine the preconditioning of the magnetosphere and the eventual destruction of the SMC state, Figures 6 and 5 display pre-SMC and post-SMC intervals in a format similar to Figure 2. By a pre-SMC (post-SMC) interval, we mean a period of the respective SMC's duration preceding (following) the SMC. We only consider the IMF Bz distributions because at the SMC time scales the solar wind speed tends to remain fairly constant.

Figure 5.

Same as Figure 2 except for post-SMC conditions.

Figure 6.

Same as Figure 2 except for pre-SMC conditions.

[19] Comparison of Figures 2 and 6 shows that while the IMF Bz tends to be fairly similar to or slightly more negative for pre-SMC intervals than for SMC intervals, the ring current is clearly weaker (SYM-H closer to zero or positive). This indicates that the magnetosphere cannot enter the SMC mode until the ring current has enhanced sufficiently. Comparison of Figures 2 and 5, on the other hand, shows that the ring current amplitude remains approximately the same after the SMC has ended (ring current decay time is of the order of a few days [e.g., Gonzalez et al., 1994]) but IMF Bz becomes either stronger or, in some cases, weaker. Thus, it appears that a ring current of a certain amplitude can stabilize the magnetotail against a certain level of solar wind driving, but when the driving exceeds this level, a substorm will follow. The other option is that the IMF turns northward, which leads to halting of the dayside reconnection and magnetospheric convection. This is in agreement with the results of 2009a]: 57% of their SMC events ended with a substorm while for the rest of the SMCs, the activity died out.

4 Conclusions

[20] We have used 32 years of observations (1981–2012) to study the effect of the ring current for preconditioning the magnetosphere for steady convection. SMC intervals were found based on the AL index, the SYM-H index was used as a proxy for the ring current amplitude, and solar wind speed and IMF Bz for the prevailing solar wind driving. We found that

  1. [21] The ring current is enhanced for the majority of SMCs.

  2. [22] Although SMCs tend to take place during slow solar wind speeds, they can also occur during higher solar wind speeds if the ring current is intense enough.

  3. [23] The mechanism by which the ring current supports the SMC mode appears to be stabilization against near-Earth reconnection [2009].

  4. [24] Pre-SMC intervals are characterized by solar wind driving that is similar to that during SMC intervals, but a weaker ring current. This indicates that the magnetosphere cannot enter the SMC mode until the ring current has enhanced sufficiently.

  5. [25] Post-SMC intervals are characterized by ring current conditions that are similar to those during SMC intervals, but either stronger or weaker solar wind driving. The former implies that when the driving exceeds a certain level, the ring current stabilization fails. The latter indicates that magnetospheric convection ends when the IMF turns northward.

Acknowledgments

[26] We acknowledge NASA/GSFC's Space Physics Data Facility's OMNIWeb service, and OMNI data. The work of L. Juusola was supported by the Academy of Finland project 137900.

[27] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper.