Small-scale transients in the Small-scale transients in the slow solar wind during solar slow solar wind during solar

activity minimumactivity minimum

K.E.J. Huttunen1,2, J.G. Luhmann1, J.T. Gosling3, Y. Li1, D. Larson1, P. Schroeder1, H. Elliot4, G. Petrie5, A. Vourlidas6,

A.B. Galvin7, C.T. Russell8

1 Space Sciences Laboratory, University of California, Berkeley, CA, USA2 Department of Physics, University of Helsinki, Finland3 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, USA4 South West Research Institute, San Antonio, TX, USA5 National Solar Observatory, Tuscon, Arizona, USA6 Solar Physics Branch, Naval Research Laboratory, Washington, DC , USA7 EOS Space Science Center, University of New Hampshire, Durham, United States8 Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA, USA

McNaught

Milky Way

Earth

Sun

CME

The Complete View from SECCHI Imagers

( STEREO B )

STEREOHeliosphericImaging isChallenging paradigms

HI images show ‘quiet’ solar wind activity all the way out to 1 AU

SLOW SOLAR WIND and the HPS concept

Wang et al., JGR, 2000

The slow (< 450 km/s) solar wind is connected to the last closed field lines of the coronal streamer belt and the

boundaries of the coronal holes. It occupies the same spatial domain as the so-called HPS. Are they the same entity?

Figure 1

One also expects to observe in-situ signatures of a transient slow wind component in this region, as

well as occasional sector boundaries (HCS)

(Cartoons from Wang et al., ApJ paper)

Streamer Cusp Solar Wind was identified and shown by Wang et al. to

attain slow solar wind speeds by ~30 Rs

(results from LASCO images, in Wang et al., ApJ, 2000)

‘Flux-Rope-like’ in-situ CME ejecta and opening loops where considered by Crooker and Kahler et al. to be a part of the streamer belt detritus

(plot from Yan Li)

Different styles of such transient behavior have long been considered and cartooned…all produce possibly

surviving and identifiable in-situ signatures

May produceBDEs

May producefalse polarityreversal

May produceHFDs

In this MAS MHD model corona evolving by differential rotation in the photosphere, Lionello

et al. simulated the time-dependent HSB boundary but did not propagate results to 1 AU

(figures from Lionello et al., ApJ, 2004)

Complications are added by stream interaction compressions in the same part of the solar wind

(V. Pizzo, JGR 1991)

DATA ANALYSIS: TowardUnderstanding the transientSlow Solar Wind structure

• slow solar wind during nine (low solar activity) Carrington rotations analyzed

• covers period March 4, 2007 (CR 2054) - November 4, 2007 (CR 2062)

• STEREO data primarily used, implemented by the WIND measurements

• STEREO separation increased from 1 to 40 degrees, WIND at L1

• Transient Signatures searched for included: smooth magnetic field, organized magnetic field rotations (loops, flux ropes etc.); temperature drops; beta depressions, electron anisotropy signatures

OVERVIEW• 35 transients identified from STEREO A and B

• variety of transients observed - ICME -type with several typical signatures (many sizes) - regions of clear temperature drop, but no other ICME signatures - mini flux ropes in the field

Vp

(km

/s)

ST

ER

EO

AS

TE

RE

O B

Plasma velocity from STEREO PLASTIC forperiod analyzed for this study (events marked)

Range Average Median SDV

|V| (km/s) 1.1 - 74.1 19.5 13.8 19.9

Bmax (nT) 1.6 - 18.0 5.9 5.5 3.4

d (AU) 0.018-0.23 0.097 0.072 0.062

Summary of Event Properties

• no significant change in speed across the transients

• magnetic fields about the same as the nominal solar wind value (~5nT)

• radial scale sizes range from mini flux ropes (few hundreds Earth radii, Moldwin et al., 2000) to scale sizes of a typical magnetic cloud at 1 AU (~0.25 AU)

• STEREO satellites observed same transients only in March-April period and already clear differences recorded scale sizes in longitudinal direction also small (< 10°)

STEREO BSTEREO A

Poster Figure 7

Example: CR 2054

STEREO A STEREO B

Poster Figure 8

Example: CR 2060

EXAMPLE CARRINGTON ROTATIONS EVENTS

• CR 2054 (Figure 7) - STEREO separation 1.3° - 2.8° - same transients identified, but some differences already visible - Event 1: clear temperature drop - Events 2 (Figure 9) and 3: ICME-type transients with counterstreaming electrons

• CR 2060 (Figure 8) - STEREO separation 24.8° - 30.1° - no same transients identified - Events 27 and 29: temperature drop - Event 28: ICME-type transient - Event 30: very clear and sharp temperature drop (Figure 11) - Event 31: ICME-type transient with counterstreaming electrons

STEREO A

Poster Figure 9 Poster Figure 10

STEREO A

ICME-type transient mini flux rope: high and smooth magnetic field, rotation consistent with a fluxrope, scale sizes only few hundred Re

Poster Figure 11

STEREO B STEREO B

Poster Figure 12

temperature drop transient: no CSE, nosmooth and organized magnetic fields

large scale ICME: d > 0.15 AU and Bmax > 10 nT

SUPRATHERMAL ELECTRON SIGNATURES

counterstreaming electron flow(occasional heat flux depletions at 90°)

- 11/35 (31%)- all but one event associated also with smooth and organized magnetic fields

unidirectional electron flow(occasional heat flux depletions at 90°)

- 24/35 (69%)- all temperature drop only transients and mini flux ropes associated with unidirectional heat flux

• no false polarity reversals or heat flux dropouts identified

Poster Figure 13

GONG PFSS Model Coronal open field mapping to the ecliptic suggests where the features in the in-situ observations arise

CR2054 CR2055

(model results from GONG website- G. Petrie)

- open fields

+ open fields

Helmet Streamer Belt

Source surface neutral line

Field lines that mapto near the ecliptic

Active region source

Quiet Sun source(polar hole or midlatitude)

MarAprMay

JunJulAug

SepOct

Poster Figure 14

SOLAR ORIGIN OF TRANSIENTS

• Global Oscillation Network Group (GONG) synoptic ecliptic field plots for CRs 2054-2062 (GONG website Gordon Petrie, NSO)

• no significant speed change across the transients: relatively good estimate of their solar “launch” time by tracing them from the satellite to the Sun using the constant speed

green: positive polarity red: negative polarityblue thick line: polarity inversion lineyellow arrows: estimated solar times of transients

• transients appear to cluster in the vicinity of coronal source surface sector boundary crossings

• footpoints of newly opening and closing field lines for the CRs examined (CR difference models),•Locations of approximate mapped event footpoints at photosphere

green: coronal holesred: closing fieldsBlue: opening fields

website plot courtesy of Gordon Petrie, NSO

CR 2054 and 2055

CR 2060 and 2061

Figure 14

SUMMARY AND CONCLUSIONS

• 35 transients were observed during CRs 2054 - 2060 (about four per CR in the slow solar wind)

• only two large-scale (d > 0.15, B > 10 nT) ICMEs observed in this period • on average no change in the solar wind speed, magnetic field values close to the nominal solar wind value. Thus not easily identified from the ambient solar wind, just seem to float with the slow solar wind main defining signatures: temperature drops, counterstreaming electrons

• radial scale sizes range from mini flux ropes (few hundred Re) to sizes of a typical magnetic cloud near 1 AU (0.25 AU)

• Combined observations by STEREO and Wind satellites suggests that longitudinal scale sizes small (< 10°)

SUMMARY AND CONCLUSIONS (cont.)

• same CRs appear to be active at both STEREO satellites even when their angular separation increases

• suprathermal electron signatures include: - counterstreaming electrons (stretching coronal loops) - unidirectional electrons (interchange reconnection?)

• their constant speed allows mapping of the transients reliably back to the Sun

• most transients seem to cluster near the coronal source surface sector boundaries

OPEN QUESTIONS AND FUTURE WORK

• what percentage of solar wind is of transient nature? What is the HPS?

• how do transients relate to the coronal opening/closing field lines? (mapping to more accurate than monthly changes needed)

• different “types” of transients: same or different origin?

• elemental composition from the STEREO satellites can be added

• When separation of the STEREO satellites is ~90° comparisons between imaged coronal blobs and in-situ measurements possible