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The Heliospheric Plasma Sheet

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The Heliospheric Plasma Sheet The Heliospheric Plasma Sheet Definition Dynamics Solar Connections Concept The HPS is a high-density sheath surrounding the heliospheric current sheet Analogous to the magneto- spheric plasma sheet Problems? What maintains high density? Not closed fields, as in magnetosphere Apparently current sheet not required Proposed Observational Definitions The heliospheric plasma sheet is the coronal streamer belt [Borrini et al., 1981; Gosling et al., 1981] the low-entropy interstream flow [Burlaga et al., 1990] a high-beta structure [Winterhalter et al., 1994; Crooker et al., 1996, 2004] an entropy hole [Neugebauer et al., 2002, 2004] All are high-density structures Timescales vary from minutes to days Coronal Streamer Belt Superposed epoch analysis centered on heliospheric current sheet First identification of high-density plasma sheet separate from compressive stream interaction Temperature, speed, He++/H+ minima at density maximum Dissimilar profiles Speed flattens for ~ 2+ days Density peaks sharply Hints at two scale sizes Superposed epoch analysis hides substantial variablility Gosling et al. [1981] Low-Entropy Interstream Flow 1995 Wind data:P a Entropygel et al. [2004] First proposed as HPS by Burlaga et al. [1990] Bound by stream interfaces on leading (red) and trailing edges of high-speed flow γ-1 Entropy ∝Tp/n is excellent interface marker Entropy anti- correlates well with O7+/O6+ This plasma sheet definition ≡ “slow wind” with elevated O7+/O6+ and Mg/O from Pagel et al. [2004], after Burlaga et al. [1990] and BurtGoeni sest eatl .a [l1. 9[19999]5] Slow Wind Scale Size Near Minimum Misleading plot of Ulysses speed data Slow wind does not span 45° near solar minimum Slow wind confined to ~half that span, like width of helmet streamer base 45°-sweep created by tilt of streamer belt relative to heliographic equator as Sun rotates thru 360°. Slow Wind Scale Size Near Minimum Synoptic map of solar wind speed Based upon Wind and Ulysses data during fast latitude scan Speed is organized by heliomagnetic than heliographic coordinates Speeds < 500 km/s span ~25° of latitude at any given longitude, even though swath spanning latitudinal extrema covers 45° Speed Contours Crooker et al. [1997] High-Beta HPS First proposed by Winterhalter et al. [“The Heliospheric Plasma Sheet,” JGR, 1994] High-beta pressure balance structure 110 minutes long Classifies as magnetic hole If T is low, can also be entropy hole [Neugebauer et al., 2002, 2004] Misaligned current hints at high variability Winterhalter et al. [1994] Heliospheric Plasma Sheet with no Heliospheric Current Sheet Arises between streams with like polarity [Neugebauer et al., 2004] Has both large- and small-scale plasma sheet characteristics No heliospheric current sheet required Can be traced back to pseudostreamers [Wang et al., 2007a,b] toward away 2002 Double-Scale Steady-State View Bavassano et al. [1997] view accommodates two scale sizes Based upon in situ and remote scintillation data Propose that HPS consists of streamer stalk surrounded by halo extending radially from base of streamer belt View serves as useful basis for further understanding Does not take into account high degree of variability Small-Scale HPS Variability Sector boundary offset from plasma sheet by >2 hrs Complicated interplay between n, T, and B Narrow magnetic hole (~1 hr) Broad n and T profiles (~6 hrs) yield broad entropy hole (not shown) T elevated in high- beta plasma sheet More Small-Scale HPS Variability Some sector boundaries (as identified in electron data) have no plasma sheet Some are widely displaced from current sheet Identifying Sector Boundaries Definition: A sector boundary separates fields of opposite solar polarity Direction of electron heat flux relative to the field distinguishes sector boundaries from localized current sheets Most current sheets across which field reverses are localized [Szabo et al., 1999] sectorsector Early studies restricted to boundaryboundary magnetometer data mistakenly concluded that HCS is extremely corrugated localized current sheets Some sector boundaries lack a current sheet because adjacent field is inverted Search for Plasma Sheets at Sector Boundaries 52 successive sector boundaries were surveyed for high-beta plasma sheets (magnetic holes) and current sheets Only 26 had both Conclusion: The view that the sector boundary is embedded in a high-beta plasma sheet applies only half of the time Crooker et al. [2004] Similar Variability at HPS with no HCS toward entropy hole away high-beta plasma sheet High-beta plasma with trailing entropy hole Reflection of complicated interplay between n, T, and B Entropy hole has different composition [M. Neugebauer] and may be pseudostreamer material Field inverts in high-beta plasma sheet Note correlation between beta and electron isotropy, as also occurs at sector boundaries High-beta PS ≡ Heat Flux Dropout Typical heat-flux dropout (isotropy) at sector boundary Heat-flux dropout may be signature of disconnection (or interchange reconnection) Multiple field reversals Elevated but variable beta Coincides with A(He) depression, as in super- posed epoch analysis Beta variability positively correlated with A(He) Suggests time-dependent release at variable solar altitude Is reconnection source of plasma sheet variability? Documented Reconnection Signatures are Subclass of High-Beta Plasma Sheets Signatures of in situ disconnection have been identified by Gosling et al. [2007] based upon accelerated, Alfvenic exhaust (not seen in plasma sheets) Common to plasma sheets and exhausts are heat-flux dropout magnetic hole Adding Reconnection to Double-Scale View —Small-Scale HPS From Sheeley, Wang et al. [1997-2000] “With the increased…sensitivity allowed by LASCO…, it has become evident that outflows…are not confined to CMEs but occur continually….” “Time-lapse sequences… indicate that streamers are far more dynamic than was previously thought, with material continually being ejected at their cusps and accelerating outward along their stalks.” These authors propose that the entire small-scale HPS consists of discontinuous blobs released by interchange reconnection Their solar observations are consistent with in situ high variability and reconnection- like signatures Wang et al. [2000] Reconnection with no Sector Boundary? - Wang et al. [2007] note outflows and no current sheet possibility of interchange + reconnection at X point at base of pseudostreamers - Calculated by Yi-M. Wang for CR 2002 pseudostreamer field viewed from N pole Streamer Source: Alternative View? Eselevich and Eselevich [2006a,b] note that quasi- steady streamer rays of variable brightness extend down to solar surface lie to either side rather than centered on current sheet can account for misalignment of density peaks and current sheet Solar eclipse and LASCO data, 11 August 1999 Streamer Source: Alternative View? Contrasts with plasma source at tip of helmet streamer as drawn by Bavassano et al. [1997] and Wang et al. [2000] Similar to steady-state view of Strachan et al. [2002] based upon UVCS measurements Synthesis may lie in idea of Wang et al. [1998, 2000] that rays are legs of newly-opened flux tubes continuing to release plasma from helmet streamer on longer time scales Consistent with brightness variability in rays Consistent with higher A(He) in high-beta plasma sheets Eselevich and Eselevich [2006] Adding Reconnection to Double-Scale View —Large-Scale HPS Problem of source of large-scale HPS is problem of source of slow wind Wang et al. [2000] show steady-state flow from coronal hole boundary, Wang et al. [2000] slow because of large expansion factor Could be fanning out of newly reconnected field lines If so, double scale is created by same process Adding Reconnection to Double-Scale View —Large-Scale HPS Fisk et al. [1999], Schwadron et al. [2005], and Fisk and Zurbuchen [2007] propose continuous interchange reconnection with large loops effects flux transport through the streamer belt to complete global circulation in the Fisk model can account for composition and charge- state variations compatible with Wang et al. [2000]? Interim Recap HPS Small-Scale Large-Scale Definition Entropy holes Slow wind marked by low Magnetic holes, HFDs entropy, high O7+/O6+, Current sheet not required Mg/O, etc. Streamer/pseudostreamer Large expansion factor Dynamics* extension and/or interchange and/or interchange reconnection/disconnection reconnection Solar Streamer/pseudostreamer Coronal hole boundaries Source outflow or large loops *Factors favoring reconnection High variability Similarity to documented in situ reconnection signatures Discontinuous nature of streamer outflows Composition and charge-state signatures Dynamics: HPS and CMEs Since CMEs do not arise from coronal holes, it follows that their heliospheric corridor is the large-scale HPS Wang et al. [2000] suggest that CMEs are part of a Transient continuum of outflow from the streamer belt Obvious near solar minimum Large body of literature, both observational and theoretical, identifies streamer belt as source of most CMEs Near solar maximum, however, there is another source: pseudostreamers CMEs from Pseudostreamers At solar maximum, pseudo- streamers are as common as helmet streamers [Zhao - and Webb, 2003] no current sheet CMEs from pseudo- streamers have been documented by + Fainshtein [1997] Eselevich et al. [1999] Zhao and Webb [2003] Liu and Hayashi [2006] (2003 Halloween
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