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Register a new userSuzaku Detection of Solar Wind Charge Exchange Emission from a Variety of Highly-ionized Ions in an Interplanetary Coronal Mass Ejection
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X-ray emission generated through solar-wind charge exchange (SWCX) is known to contaminate X-ray observation data, the amount of which is often significant or even dominant, particularly in the soft X-ray band, when the main target is comparatively weak diffuse sources, depending on the space weather during the observation. In particular, SWCX events caused by interplanetary coronal mass ejections (ICMEs) tend to be spectrally rich and to provide critical information about the metal abundance in the ICME plasma. We analyzed the SN1006 background data observed with Suzaku on 2005 September 11 shortly after an X6-class solar flare, signatures of which were separately detected together with an associated ICME. We found that the data include emission lines from a variety of highly ionized ions generated through SWCX. The relative abundances of the detected ions were found to be consistent with those in past ICME-driven SWCX events. Thus, we conclude that this event was ICME-driven. In addition, we detected a sulfur XVI line for the first time as one from the SWCX emission, the fact of which suggests that it is the most spectrally-rich SWCX event ever observed. We suggest that observations of ICME-driven SWCX events can provide a unique probe to study the population of highly-ionized ions in the plasma, which is difficult to measure in currently-available in-situ observations.
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Suzaku detected an enhancement of soft X-ray background associated with solar eruptions on 2013 April 14-15. The solar eruptions were accompanied by an M6.5 solar flare and a coronal mass ejection with magnetic flux ropes. The enhanced soft X-ray background showed a slight variation in half a day and then a clear one in a few hours. The former spectrum was composed of oxygen emission lines, while the later one was characterized by a series of emission lines from highly ionized carbon to silicon. The soft X-ray enhancement originated from geocoronal solar wind charge exchange. However, there appeared to be no significant time correlation with the solar wind proton flux measured by the ACE and WIND satellites. From other solar wind signatures, we considered that an interplanetary shock associated with the coronal mass ejection and a turbulent sheath immediately behind the shock compressed the ambient solar wind ions and then resulted in the soft X-ray enhancement. Furthermore, the enriched emission lines were presumed to be due to an unusual set of ion abundances and ionization states within the coronal mass ejection. We found a better time correlation with the solar wind alpha flux rather than the solar wind proton flux. Our results suggest that the solar wind proton flux is not always a good indicator of geocoronal solar wind charge exchange, especially associated with coronal mass ejections. Instead, the solar wind alpha flux should be investigated when such a soft X-ray enhancement is detected in astronomical observations.
Coronal mass ejections (CMEs) are large-scale eruptions of magnetized plasma that may cause severe geomagnetic storms if Earth-directed. Here we report a rare instance with comprehensive in situ and remote sensing observa- tions of a CME combining white-light, radio, and plasma measurements from four different vantage points. For the first time, we have successfully applied a radio direction-finding technique to an interplanetary type II burst detected by two identical widely separated radio receivers. The derived locations of the type II and type III bursts are in general agreement with the white light CME recon- struction. We find that the radio emission arises from the flanks of the CME, and are most likely associated with the CME-driven shock. Our work demon- strates the complementarity between radio triangulation and 3D reconstruction techniques for space weather applications.
Fast interplanetary coronal mass ejections (interplanetary CMEs, or ICMEs) are the drivers of strongest space weather storms such as solar energetic particle events and geomagnetic storms. The connection between space weather impacting solar wind disturbances associated with fast ICMEs at Earth and the characteristics of causative energetic CMEs observed near the Sun is a key question in the study of space weather storms as well as in the development of practical space weather prediction. Such shock-driving fast ICMEs usually expand at supersonic speed during the propagation, resulting in the continuous accumulation of shocked sheath plasma ahead. In this paper, we propose the sheath-accumulating propagation (SAP) model that describe the coevolution of the interplanetary sheath and decelerating ICME ejecta by taking into account the process of upstream solar wind plasma accumulation within the sheath region. Based on the SAP model, we discussed (1) ICME deceleration characteristics, (2) the fundamental condition for fast ICME at Earth, (3) thickness of interplanetary sheath, (4) arrival time prediction and (5) the super-intense geomagnetic storms associated with huge solar flares. We quantitatively show that not only speed but also mass of the CME are crucial in discussing the above five points. The similarities and differences among the SAP model, the drag-based model and the`snow-plough model proposed by citet{tappin2006} are also discussed.
In situ measurements of interplanetary coronal mass ejection (ICME) composition, including elemental abundances and charge states of heavy ions, open a new avenue to study coronal mass ejections (CMEs) besides remote-sensing observations. The ratios between different elemental abundances can diagnose the plasma origin of CMEs (e.g., from the corona or chromosphere/photosphere) due to the first ionization potential (FIP) effect, which means elements with different FIP get fractionated between the photosphere and corona. The ratios between different charge states of a specific element can provide the electron temperature of CMEs in the corona due to the freeze-in effect, which can be used to investigate their eruption process. In this review, we first give an overview of the ICME composition and then demonstrate their applications in investigating some important subjects related to CMEs, such as the origin of filament plasma and the eruption process of magnetic flux ropes. Finally, we point out several important questions that should be addressed further for better utilizing the ICME composition to study CMEs.
We present results from a sample of XMM-Newton and Suzaku observations of interstellar clouds that cast shadows in the soft X-ray background (SXRB) - the first uniform analysis of such a sample from these missions. By fitting to the on- and off-shadow spectra, we separated the foreground and Galactic halo components of the SXRB. We tested different foreground models - two solar wind charge exchange (SWCX) models and a Local Bubble (LB) model. We also examined different abundance tables. We found that Anders & Grevesse (1989) abundances, commonly used in previous SXRB studies, may result in overestimated foreground brightnesses and halo temperatures. We also found that assuming a single solar wind ionization temperature for a SWCX model can lead to unreliable results. We compared our measurements of the foreground emission with predictions of the SWCX emission from a smooth solar wind, finding only partial agreement. Using available observation-specific SWCX predictions and various plausible assumptions, we placed an upper limit on the LBs OVII intensity of ~0.8 photons/cm^2/s/sr (90% confidence). Comparing the halo results obtained with SWCX and LB foreground models implies that, if the foreground is dominated by SWCX and is brighter than ~1.5e-12 erg/cm^2/s/deg^2 (0.4-1.0 keV), then using an LB foreground model may bias the halo temperature upward and the 0.5-2.0 keV surface brightness downward by ~(0.2-0.3)e6 K and ~(1-2)e-12 erg/cm^2/s/deg^2, respectively. Similarly, comparing results from different observatories implies that there may be uncertainties in the halo temperature and surface brightness of up to ~0.2e6 K and ~25%, respectively, in addition to the statistical uncertainties. These uncertainties or biases may limit the ability of X-ray measurements to discriminate between Galactic halo models.
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