Interplanetary Coronal Mass Ejections (ICMEs), their possible shocks and sheaths, and co-rotating interaction regions (CIRs) are the primary large-scale heliospheric structures driving geospace ...disturbances at the Earth. CIRs are followed by a faster stream where Alfvénic fluctuations may drive prolonged high-latitude activity. In this paper we highlight that these structures have all different origins, solar wind conditions and as a consequence, different geomagnetic responses. We discuss general solar wind properties of sheaths, ICMEs (in particular those showing the flux rope signatures), CIRs and fast streams and how they affect their solar wind coupling efficiency and the resulting magnetospheric activity. We show that there are two different solar wind driving modes: (1) Sheath-like with turbulent magnetic fields, and large Alfvén Mach (
M
A
) numbers and dynamic pressure, and (2) flux rope-like with smoothly varying magnetic field direction, and lower
M
A
numbers and dynamic pressure. We also summarize the key properties of interplanetary shocks for space weather and how they depend on solar cycle and the driver.
Coronal mass ejections (CMEs) are one of the primary manifestations of solar activity and can drive severe space weather effects. Therefore, it is vital to work towards being able to predict their ...occurrence. However, many aspects of CME formation and eruption remain unclear, including whether magnetic flux ropes are present before the onset of eruption and the key mechanisms that cause CMEs to occur. In this work, the pre-eruptive coronal configuration of an active region that produced an interplanetary CME with a clear magnetic flux rope structure at 1 AU is studied. A forward-S sigmoid appears in extreme-ultraviolet (EUV) data two hours before the onset of the eruption (SOL2012-06-14), which is interpreted as a signature of a right-handed flux rope that formed prior to the eruption. Flare ribbons and EUV dimmings are used to infer the locations of the flux rope footpoints. These locations, together with observations of the global magnetic flux distribution, indicate that an interaction between newly emerged magnetic flux and pre-existing sunspot field in the days prior to the eruption may have enabled the coronal flux rope to form via tether-cutting-like reconnection. Composition analysis suggests that the flux rope had a coronal plasma composition, supporting our interpretation that the flux rope formed via magnetic reconnection in the corona. Once formed, the flux rope remained stable for two hours before erupting as a CME.
We use Van Allen Probes data to investigate the responses of tens of keV to 2 MeV electrons throughout a broad range of the radiation belts (2.5 ≤ L ≤ 6.0) during 52 geomagnetic storms from the most ...recent solar maximum. Electron storm time responses are highly dependent on both electron energy and L shell. Tens of keV electrons typically have peak fluxes in the inner belt or near‐Earth plasma sheet and fill the inner magnetosphere during storm main phases. Approximately 100 to ~600 keV electrons are enhanced in up to 87% of cases around L~3.7, and their peak flux location moves to lower L shells during storm recovery phases. Relativistic electrons (≥~1 MeV) are nearly equally likely to produce enhancement, depletion, and no‐change events in the outer belt. We also show that the L shell of peak flux correlates to storm magnitude only for hundreds of keV electrons.
Key Points
Near L~3.7, ~100 to ~600 keV electrons are enhanced during up to 87% of storm recovery phases
Relativistic electrons are split between enhancement, depletion, and no‐change events
For hundreds of keV electrons, peak flux L shell after the storm correlates well to storms' SYM‐H minima
Aims. We present a detailed study of the magnetic evolution of AR 12673 using a magnetofrictional modelling approach. Methods. The fully data-driven and time-dependent model was driven with maps of ...the photospheric electric field, inverted from vector magnetogram observations obtained from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Our analysis was aided by studying the evolution of metrics such as the free magnetic energy and the current-carrying helicity budget of the domain, maps of the squashing factor and twist, and plots of the current density. These allowed us to better understand the dynamic nature of the magnetic topology. Results. Our simulation captured the time-dependent nature of the active region and the erupting flux rope associated with the X-class flares on 6 September 2017, including the largest of solar cycle 24. Additionally, our results suggest a possible threshold for eruptions in the ratio of current-carrying helicity to relative helicity. Conclusion. The flux rope was found to be a combination of two structures that partially combine during the eruption process. Our time-dependent data-driven magnetofrictional model is shown to be capable of generating magnetic fields consistent with extreme ultraviolet (EUV) observations.
Interplanetary coronal mass ejections (ICMEs) are a significant feature of the heliospheric environment and the primary cause of adverse space weather at the Earth. ICME propagation and the evolution ...of ICME magnetic field structure during propagation are still not fully understood. We analyze the magnetic field structures of 18 ICME magnetic flux ropes observed by radially aligned spacecraft in the inner heliosphere. Similarity in the underlying flux rope structures is determined through the application of a simple technique that maps the magnetic field profile from one spacecraft to the other. In many cases, the flux ropes show very strong underlying similarities at the different spacecraft. The mapping technique reveals similarities that are not readily apparent in the unmapped data and is a useful tool when determining whether magnetic field time series observed at different spacecraft are associated with the same ICME. Lundquist fitting has been applied to the flux ropes, and the rope orientations have been determined; macroscale differences in the profiles at the aligned spacecraft may be ascribed to differences in flux rope orientation. Assuming that the same region of the ICME was observed by the aligned spacecraft in each case, the fitting indicates some weak tendency for the rope axes to reduce in inclination relative to the solar equatorial plane and to align with the solar east‐west direction with heliocentric distance.
Plain Language Summary
Coronal mass ejections (CMEs) are large eruptions of magnetic field and plasma from the Sun. When they arrive at the Earth, these eruptions can cause significant damage to ground and orbital infrastructure; forecasting this “space weather” impact of CMEs at the Earth remains a difficult task. The impact of individual CMEs is largely dependent on their magnetic field configurations, and an important aspect of space weather forecasting is understanding how CME field configuration changes with distance from the Sun. We have analyzed the signatures of 18 CMEs observed by pairs of lined‐up spacecraft and show that their basic magnetic field structures often display significant self‐similarities, that is, they do not often show significant reordering of field features with heliocentric distance. This similarity points to the general usefulness of placing spacecraft between the Sun and Earth to act as early‐warning space weather monitors. CME signatures observed at such monitors would likely be similar to the signatures subsequently arriving at the Earth and could be used to produce space weather forecasts with longer lead times.
Key Points
Eighteen interplanetary flux ropes observed by radially aligned spacecraft in the inner heliosphere have been examined
Many of the flux ropes showed significant self‐similarities in magnetic field structure at the aligned spacecraft
Macroscale differences in the magnetic field profiles are consistent with the flux ropes displaying different axis orientations
Flux ropes ejected from the Sun may change their geometrical orientation during their evolution, which directly affects their geoeffectiveness. Therefore, it is crucial to understand how solar flux ...ropes evolve in the heliosphere to improve our space-weather forecasting tools. We present a follow-up study of the concepts described by Isavnin, Vourlidas, and Kilpua (
Solar Phys.
284
, 203,
2013
). We analyze 14 coronal mass ejections (CMEs), with clear flux-rope signatures, observed during the decay of Solar Cycle 23 and rise of Solar Cycle 24. First, we estimate initial orientations of the flux ropes at the origin using extreme-ultraviolet observations of post-eruption arcades and/or eruptive prominences. Then we reconstruct multi-viewpoint coronagraph observations of the CMEs from ≈ 2 to 30 R
⊙
with a three-dimensional geometric representation of a flux rope to determine their geometrical parameters. Finally, we propagate the flux ropes from ≈ 30 R
⊙
to 1 AU through MHD-simulated background solar wind while using
in-situ
measurements at 1 AU of the associated magnetic cloud as a constraint for the propagation technique. This methodology allows us to estimate the flux-rope orientation all the way from the Sun to 1 AU. We find that while the flux-ropes’ deflection occurs predominantly below 30 R
⊙
, a significant amount of deflection and rotation happens between 30 R
⊙
and 1 AU. We compare the flux-rope orientation to the local orientation of the heliospheric current sheet (HCS). We find that slow flux ropes tend to align with the streams of slow solar wind in the inner heliosphere. During the solar-cycle minimum the slow solar-wind channel as well as the HCS usually occupy the area in the vicinity of the solar equatorial plane, which in the past led researchers to the hypothesis that flux ropes align with the HCS. Our results show that exceptions from this rule are explained by interaction with the Parker-spiraled background magnetic field, which dominates over the magnetic interaction with the HCS in the inner heliosphere at least during solar-minimum conditions.
We present a comprehensive statistical analysis spanning over a solar cycle of the properties and drivers of traveling fast forward and fast reverse interplanetary shocks. We combine statistics of ...679 shocks between 1995 and 2013 identified from the near‐Earth (Wind and ACE) and STEREO‐A observations. We find that fast forward shocks dominate over fast reverse shocks in all solar cycle phases except during solar minimum. Nearly all fast reverse shocks are driven by slow‐fast stream interaction regions (SIRs), while coronal mass ejections (CMEs) are the principal drivers of fast forward shocks in all phases except at solar minimum. The occurrence rate and median speeds of CME‐driven fast forward shocks follow the sunspot cycle, while SIR‐associated shocks do not show such correspondence. The strength of the shock (characterized by the magnetosonic Mach number and by the upstream to downstream magnetic field and density ratio) shows relatively little variations over solar cycle. However, the shocks were slightly stronger during the ascending phase of a relatively weak solar cycle 24 than during the previous ascending phase. The CME‐ and SIR‐driven fast forward shocks and fast reverse shocks have distinct upstream solar wind conditions, which reflect to their relative strengths. We found that CME‐driven shocks are on average stronger and faster, and they show broader distributions of shock parameters than the shocks driven by SIRs.
Key Points
First study on IP shock properties and drivers spanning over a solar cycle
Shock strength does not vary with solar cycle; speed and rate have a clear trend
Shock strength and parameter distributions depend clearly on the driver
We identify all fast‐mode forward shocks, whose sheath regions resulted in a moderate (56 cases) or intense (38 cases) geomagnetic storm during 18.5 years from January 1997 to June 2015. We study ...their main properties, interplanetary causes, and geoeffects. We find that half (49/94) such shocks are associated with interacting coronal mass ejections (CMEs), as they are either shocks propagating into a preceding CME (35 cases) or a shock propagating into the sheath region of a preceding shock (14 cases). About half (22/45) of the shocks driven by isolated transients and which have geoeffective sheaths compress preexisting southward Bz. Most of the remaining sheaths appear to have planar structures with southward magnetic fields, including some with planar structures consistent with field line draping ahead of the magnetic ejecta. A typical (median) geoeffective shock‐sheath structure drives a geomagnetic storm with peak Dst of −88 nT pushes the subsolar magnetopause location to 6.3 RE, i.e., below geosynchronous orbit and is associated with substorms with a peak AL index of −1350 nT. There are some important differences between sheaths associated with CME‐CME interaction (stronger storms) and those associated with isolated CMEs (stronger compression of the magnetosphere). We detail six case studies of different types of geoeffective shock‐sheaths, as well as two events for which there was no geomagnetic storm but other magnetospheric effects. Finally, we discuss our results in terms of space weather forecasting and potential effects on Earth's radiation belts.
Key Points
Forty‐nine shocks/sheaths in a previous transient and 45 isolated shocks drove moderate or intense geomagnetic storms from 1997 to 2015
Fifty percent of shocks in transients and 13% of shocks in normal solar wind had a sheath region that drove at least a moderate storm
A median geoeffective shock/sheath pushed the magnetopause to 6.3 RE, had strong substorms (AL = ‐1350 nT) and moderate storm (Dst = ‐88 nT)
A key aim in space weather research is to be able to use remote-sensing observations of the solar atmosphere to extend the lead time of predicting the geoeffectiveness of a coronal mass ejection ...(CME). In order to achieve this, the magnetic structure of the CME as it leaves the Sun must be known. In this article we address this issue by developing a method to determine the intrinsic flux rope type of a CME solely from solar disk observations. We use several well-known proxies for the magnetic helicity sign, the axis orientation, and the axial magnetic field direction to predict the magnetic structure of the interplanetary flux rope. We present two case studies: the 2 June 2011 and the 14 June 2012 CMEs. Both of these events erupted from an active region, and despite having clear
in situ
counterparts, their eruption characteristics were relatively complex. The first event was associated with an active region filament that erupted in two stages, while for the other event the eruption originated from a relatively high coronal altitude and the source region did not feature a filament. Our magnetic helicity sign proxies include the analysis of magnetic tongues, soft X-ray and/or extreme-ultraviolet sigmoids, coronal arcade skew, filament emission and absorption threads, and filament rotation. Since the inclination of the post-eruption arcades was not clear, we use the tilt of the polarity inversion line to determine the flux rope axis orientation and coronal dimmings to determine the flux rope footpoints, and therefore, the direction of the axial magnetic field. The comparison of the estimated intrinsic flux rope structure to
in situ
observations at the Lagrangian point L1 indicated a good agreement with the predictions. Our results highlight the flux rope type determination techniques that are particularly useful for active region eruptions, where most geoeffective CMEs originate.
Abstract
Solar activities, in particular coronal mass ejections (CMEs), are often accompanied by bursts of radiation at meter wavelengths. Some of these bursts have a long duration and extend over a ...wide frequency band, namely, type IV radio bursts. However, the association of type IV bursts with CMEs is still not well understood. In this article, we perform the first statistical study of type IV solar radio bursts in solar cycle 24. Our study includes a total of 446 type IV radio bursts that occurred during this cycle. Our results show that a clear majority, ∼81% of type IV bursts, were accompanied by CMEs, based on a temporal association with white-light CME observations. However, we found that only ∼2.2% of the CMEs are accompanied by type IV radio bursts. We categorized the type IV bursts as moving or stationary based on their spectral characteristics and found that only ∼18% of the total type IV bursts in this study were moving type IV bursts. Our study suggests that type IV bursts can occur with both “Fast” (≥500 km s
−1
) and “Slow” (<500 km s
−1
), and also both “Wide” (≥60°) and “Narrow” (<60°), CMEs. However, the moving type IV bursts in our study were mostly associated with “Fast” and “Wide” CMEs (∼52%), similar to type II radio bursts. Contrary to type II bursts, stationary type IV bursts have a more uniform association with all CME types.
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