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Radio shocks from reconnection outflow jet?—New observations
RADIO JET ?
SHOCKS FROM RECONNECTION NEW O B S E R V A T I O N S
OUTFLOW
H. Aurass 1, M. Karlicky 2, B. J. Thompson 3, and B. Vr~nak 4
1Astrophysikalisches Institut Potsdam, An der Sternwarte 16,D-1~82 Potsdam, Germany 2Astronomical Institute, CZ-251 65 Ond~ejov, Czech Republic 3NASA Goddard Space Flight Center, code 682.3, Greenbelt, MD-20771, USA 4Hvar Observatory, University of Zagreb, Ka~icdva 26, HR-10000 Zagreb, Croatia
ABSTRACT We point out possible radio signatures of reconnection outfow termination shocks during the impulsive flare phase. The standing shock signature is identified between 350 and 800 MHz during the four minutes of most energetic hard X-ray emission. SOHO-EIT images in the corresponding time interval reveal a flare loop, the growth of a hot cusp-shaped arcade, and colder post-flare loops underneath.
INTRODUCTION Models of dynamic (two-ribbon, arcade) flares involve the formation of a system of standing slow and possibly also fast mode shock waves associated with the fast reconnection process below the erupting filament. We have sketched the plasma-magnetic field configuration in Figure la. The inflowing plasma (arrows pointing toward the diffusion region, DR) is squeezed between the merging field line systems and is ejected at high velocity along the thin current sheet. Two pairs of slow mode standing MHD shocks (SMSS) are formed extending from the DR and separating the inflow and outflow plasma regions. The SMSSs are sites of strong electric currents (e.g. Sato & Hayashi 1979) with a current density comparable to the DR conditions. They are a potential source of nonthermal particles. These particles can be trapped between the SMSSs. As sketched in Figure la both reconnection jets are directed toward an obstacle (EP and PFL in Figure la). If the outflow jet is supermagnetosonic then most probably above the postflare loops a standing fast mode MHD shock will be formed. Such shocks can under certain circumstances excite a non-drifting type II burst. Aurass, Vr~nak & Mann (2001, Paper I henceforth) recognized for the first time the radio signature of a fast mode outflow termination shock in a dynamic radio burst spectrogram. It started almost 1 hour after the impulsive phase and lasted for more than 30 min. Simultaneous imaging observations showed a postflare loop arcade with a bright soft X-ray cusp. In Paper I favorable circumstances for the radio detection of a termination shock in the reconnection outflow were emphasized to be a comparatively large height of the
diffusion region, a low plasma to magnetic pressure ratio ~ upstream of the slow shocks, and a small angle between the reconnecting field lines. We searched for nondrifting radio features with the typical HerringBone fine structure (HB, see Nelson & Melrose 1985) in the same frequency range as discussed in Paper I but during the impulsive flare phase. As an unique indicator of this time interval we used data of the new Czech hard X-ray spectrometer (HXRS) aboard the US MTI satellite (Farnik et al. 2001). The information about the geometry of coronal plasma structures is attributed in the given event by the Extreme Ultraviolet Telescope aboard the Solar Heliospheric Observatory (SOHO-EIT, Delaboudini~re et al. 1995). A rough -401 -
H. Aurass et al.
Fig. 1. a) Flare plasma-magnetic field configuration. DR-Diffusion Region; EP-Erupting Prominence; SMSSSlow Mode Standing Shocks; thick arrows-hot outflowing jets; FMSS-Fast Mode Standing Shock; PFLPostFlare Loops; Ha-flare ribbons, b) 29 March 2001, AR 9393, S O H O - E I T images (195 ~,-1.6.106 K) showing cusp evolution--the part of a) underneath DR--during the impulsive phase.
inspection of Nanqay Radio Heliograph data 1 confirmed that the radio sources are situated in and around AR 9393. We give a brief description and discussion of this set of observations. THE 29 MARCH 2001 E V E N T - OBSERVATIONS On 29 March 2001 there was a 1N, X1.7 flare reported in AR9393 (N16W12) between 09:55 and 11:25 UT (NOAA Solar Geophysical Data, March 2001). S O H O - E I T images reveal that the preflare activation started not later than 09:24 UT. During the event also neighboring active regions are involved (NOAA AR 9401, 9394, 9395, 9405). Figure lb shows the two images which are available at the beginning and near the end of the impulsive flare phase. In the 195 A emission it is nicely seen how a previously visible flare loop starts to deform to a more and more height-extended cusp-shaped structure. Due to the favorable perspective we see directly the structure as expected from Figure la, the range below DR. During the flare, the two southward situated active regions are disturbed by the arriving flare wave at 10:13 UT (not shown in our Figure lb). Simultaneously, a preflare noise storm radio continuum, and most of the radio flare components are suppressed (see Chertok et al. 2001) as recorded in the 40-800 MHz radio spectral data of Astrophysical Institute Potsdam (AIP). In Figure 2a we show single frequency cuts through the radio spectrum at 327 MHz (flare burst) and 70 MHz (continuum with flare-induced depression). In addition, Figure 2a gives the HXR flux of the flare in three channels between 12.6 and 147.2 keV. The most powerful energy release is detected in two enhancements between 10:04 and 10:12 UT with a minimum at 10:08 UT. Further, the 3 GHz radio flux (AO Ondrejov) is overplotted. The coronagraph aboard S O H O reveals a halo CME ejected with 942 kms -1 and a (linearly) backwards extrapolated starting time of about 09:40 UT 2. As expected (Aurass et al. 1999), in the estimated onset time interval (09:43 UT) some drift bursts are observed between 300 and 400 MHz. In the radio spectrum, Figure 2b, three slowly drifting bursts are evident between 400 and 70 MHz. The Fundamental-Harmonic (F-H) mode distinction of these bursts is difficult due to off-scale flux at lower frequencies and ongoing noise storm continuum. From 300 to 100 MHz, there is a faint type II burst without F-H-pattern between 09:58 and 10:02 UT yielding a speed of 980 kms -1 if we assume F-mode emission and iftp://mesola, obspm, fr/j rh_film/ 2http ://cdaw. gsfc. nasa. gov/CME_list/index, html
- 402 -
Radio Shocks from Reconnection Ouoqow J e t ? - New Observations
Fig. 2. Hard X-ray spectrometer (AO Ond~ejov) and radio data (AI Potsdam, AO Ond~ejov) of 29 March 2001. a) Three HXRS channels together with 3 GHz, 327 and 70 MHz plotted in arbitrary units, b) Dynamic radio spectrogram. Horizontal stripes result from strong interference. The black dashed box is enlarged in Figure 3a. Arrows point at other "HB" patches (see text). White dash-dotted line - the slowest drifting feature. a one-fold Newkirk density model. This means we find a reasonable density model transforming the burst drift rate into the independently determined CME speed. If this burst is a CME signature the delay between first faint drift bursts and the proper CME-lift-off is not unusual (Aurass et al. 1999). At 10:03- > 10:07 UT, we recognize F and H mode signatures of a "conventional" type II burst between 250 and 70 MHz. During the type II emission the noise storm starts to get depressed. Eventually, at 10:04 UT a slowly drifting and narrowband enhancement appears (190-260 k m s -1 with the Newkirk model; white dashdotted in Figure 2b). This feature resembles to the sawtooth pattern discovered by Klassen et al. (2001). From SOHO-EIT images we estimate an EIT wave speed of about 200 k m s -1 possibly corresponding to the slowest drifting radio feature. Furthermore, as shown in Figure 2b, there are patches of drift bursts between 250-800 MHz. At about 10:04:15 UT, one patch covers the starting point of the sawtooth-like slow drift burst. An F-H relation can not be recognized in these patches. But some of them seem to tend more towards lower, others more towards higher frequencies in the spectrum. An enlargement of one of these patches (Figure 3a) discloses that this can be HB emission. HB fine structure of type II bursts is due to nonthermal electrons energized at coronal shock waves. The HB patches are clearly not connected with the above mentioned low frequency ("conventional") type II burst. Because the patches itself do not show a well defined drift rate we argue for radio emission from one (or more) standing fast mode magnetosonic shocks such a sketched in Figure l a as FMSS. In Figure 3b we have plotted smoothed cuts through the HB patches at some representative frequencies. Superposed are three channels of the HXRS data. The figure demonstrates that the HB patches appear simultaneously with the HXR emission in the (most energetic) 100-147 keV channel of HXRS.
CONCLUSIONS We demonstrated the presence of patches of radio fine structures reminiscent of herringbones - a typical shock-excited radio fine structure - during the main maximum of energy release (shown by HXRS data) of a dynamic flare. This emission is observed in the decimeter range (here: 2 5 0 - > 800 MHz). As in Paper I we did not find a fundamental-harmonic mode pattern. We argue that these patches are excited by the perpendicular standing fast mode shocks sketched in Figure la. The simultaneous cusp formation and evolution (shown by SOHO-EIT data) supports our argument. Thus we extended the work started in - 403 -
H. Aurass et al.
Fig. 3. Decimetric HB emission, a) Enlargement of the dashed box in Figure 2b. Notice drift rates of both signs. b) Selected single frequency flux of smoothed HB patches (baseline shifted for clarity). Superposed (dashed) are 3 HXRS channels (compare with Figure 2a). Paper I by a well observed case of a probable decimetric standing shock signatures during the impulsive flare phase. The presented result confirms the assumption of Paper I that standing shock signatures in dynamic spectra are not rare but mostly not recognized as shock-driven emission due to insufficient sensitivity, time and frequency resolution of most sweep spectrometers, and due to previously missing supplementary observations in other spectral ranges. ACKNOWLEDGEMENTS HXRS is a joint endeavor by AICAS of the Czech Republic and SEC of NOAA, USA. The CME catalog is generated and maintained by the Center for Solar Physics and Space Weather 3. SOHO is a project of international cooperation between ESA and NASA. REFERENCES Aurass, H., A. Vourlidas, M.D. Andrews, B.J. Thompson, R.H. Howard, & G. Mann, Nonthermal Radio Signatures of Coronal Disturbances with and without CMEs, ApJ, 511, 451 (1999). Aurass, H., B. Vr~nak, & G. Mann, (Paper I), Shock-excited Radio Burst from Reconnection Outflow Jet? A 8JA, 384, 273 (2002). Chertok, I.M., S. Kahler, H. Aurass, & A.A. Gnezdilov, Sharp Decreases of Solar Metric Radio Storm Emission, Solar Phys., 202, 337 (2001). Delaboudini~re, J.P., et aL, EIT: EUV Imaging Telescope for SOHO, Solar Phys., 162, 291 (1995). Farnik, F., H. Garcia, & M. Karlicky, A New Solar Hard X-ray Spectrometer, Solar Phys., 201,357 (2001). Klassen, A., H. Aurass, & G. Mann, Sawtooth Oscillations in Solar Flare Radio Emission, A 8JA, 370, L41 (2001). Nelson, G.S. & D.B. Melrose, Type II Bursts, in Solar Radiophysics, eds. D.J. McLean, & N.R. Labrum, Cambridge Univ. Press, Cambridge, 333 (1985). Sato, T. & T. Hayashi, Externally Driven Magnetic Reconnection, Phys. Fluids, 22, 1189 (1979).
aThe Catholic University of America in cooperation with the Naval Research Laboratory and NASA. - 404 -
SHOCKS FROM RECONNECTION NEW O B S E R V A T I O N S
OUTFLOW
H. Aurass 1, M. Karlicky 2, B. J. Thompson 3, and B. Vr~nak 4
1Astrophysikalisches Institut Potsdam, An der Sternwarte 16,D-1~82 Potsdam, Germany 2Astronomical Institute, CZ-251 65 Ond~ejov, Czech Republic 3NASA Goddard Space Flight Center, code 682.3, Greenbelt, MD-20771, USA 4Hvar Observatory, University of Zagreb, Ka~icdva 26, HR-10000 Zagreb, Croatia
ABSTRACT We point out possible radio signatures of reconnection outfow termination shocks during the impulsive flare phase. The standing shock signature is identified between 350 and 800 MHz during the four minutes of most energetic hard X-ray emission. SOHO-EIT images in the corresponding time interval reveal a flare loop, the growth of a hot cusp-shaped arcade, and colder post-flare loops underneath.
INTRODUCTION Models of dynamic (two-ribbon, arcade) flares involve the formation of a system of standing slow and possibly also fast mode shock waves associated with the fast reconnection process below the erupting filament. We have sketched the plasma-magnetic field configuration in Figure la. The inflowing plasma (arrows pointing toward the diffusion region, DR) is squeezed between the merging field line systems and is ejected at high velocity along the thin current sheet. Two pairs of slow mode standing MHD shocks (SMSS) are formed extending from the DR and separating the inflow and outflow plasma regions. The SMSSs are sites of strong electric currents (e.g. Sato & Hayashi 1979) with a current density comparable to the DR conditions. They are a potential source of nonthermal particles. These particles can be trapped between the SMSSs. As sketched in Figure la both reconnection jets are directed toward an obstacle (EP and PFL in Figure la). If the outflow jet is supermagnetosonic then most probably above the postflare loops a standing fast mode MHD shock will be formed. Such shocks can under certain circumstances excite a non-drifting type II burst. Aurass, Vr~nak & Mann (2001, Paper I henceforth) recognized for the first time the radio signature of a fast mode outflow termination shock in a dynamic radio burst spectrogram. It started almost 1 hour after the impulsive phase and lasted for more than 30 min. Simultaneous imaging observations showed a postflare loop arcade with a bright soft X-ray cusp. In Paper I favorable circumstances for the radio detection of a termination shock in the reconnection outflow were emphasized to be a comparatively large height of the
diffusion region, a low plasma to magnetic pressure ratio ~ upstream of the slow shocks, and a small angle between the reconnecting field lines. We searched for nondrifting radio features with the typical HerringBone fine structure (HB, see Nelson & Melrose 1985) in the same frequency range as discussed in Paper I but during the impulsive flare phase. As an unique indicator of this time interval we used data of the new Czech hard X-ray spectrometer (HXRS) aboard the US MTI satellite (Farnik et al. 2001). The information about the geometry of coronal plasma structures is attributed in the given event by the Extreme Ultraviolet Telescope aboard the Solar Heliospheric Observatory (SOHO-EIT, Delaboudini~re et al. 1995). A rough -401 -
H. Aurass et al.
Fig. 1. a) Flare plasma-magnetic field configuration. DR-Diffusion Region; EP-Erupting Prominence; SMSSSlow Mode Standing Shocks; thick arrows-hot outflowing jets; FMSS-Fast Mode Standing Shock; PFLPostFlare Loops; Ha-flare ribbons, b) 29 March 2001, AR 9393, S O H O - E I T images (195 ~,-1.6.106 K) showing cusp evolution--the part of a) underneath DR--during the impulsive phase.
inspection of Nanqay Radio Heliograph data 1 confirmed that the radio sources are situated in and around AR 9393. We give a brief description and discussion of this set of observations. THE 29 MARCH 2001 E V E N T - OBSERVATIONS On 29 March 2001 there was a 1N, X1.7 flare reported in AR9393 (N16W12) between 09:55 and 11:25 UT (NOAA Solar Geophysical Data, March 2001). S O H O - E I T images reveal that the preflare activation started not later than 09:24 UT. During the event also neighboring active regions are involved (NOAA AR 9401, 9394, 9395, 9405). Figure lb shows the two images which are available at the beginning and near the end of the impulsive flare phase. In the 195 A emission it is nicely seen how a previously visible flare loop starts to deform to a more and more height-extended cusp-shaped structure. Due to the favorable perspective we see directly the structure as expected from Figure la, the range below DR. During the flare, the two southward situated active regions are disturbed by the arriving flare wave at 10:13 UT (not shown in our Figure lb). Simultaneously, a preflare noise storm radio continuum, and most of the radio flare components are suppressed (see Chertok et al. 2001) as recorded in the 40-800 MHz radio spectral data of Astrophysical Institute Potsdam (AIP). In Figure 2a we show single frequency cuts through the radio spectrum at 327 MHz (flare burst) and 70 MHz (continuum with flare-induced depression). In addition, Figure 2a gives the HXR flux of the flare in three channels between 12.6 and 147.2 keV. The most powerful energy release is detected in two enhancements between 10:04 and 10:12 UT with a minimum at 10:08 UT. Further, the 3 GHz radio flux (AO Ondrejov) is overplotted. The coronagraph aboard S O H O reveals a halo CME ejected with 942 kms -1 and a (linearly) backwards extrapolated starting time of about 09:40 UT 2. As expected (Aurass et al. 1999), in the estimated onset time interval (09:43 UT) some drift bursts are observed between 300 and 400 MHz. In the radio spectrum, Figure 2b, three slowly drifting bursts are evident between 400 and 70 MHz. The Fundamental-Harmonic (F-H) mode distinction of these bursts is difficult due to off-scale flux at lower frequencies and ongoing noise storm continuum. From 300 to 100 MHz, there is a faint type II burst without F-H-pattern between 09:58 and 10:02 UT yielding a speed of 980 kms -1 if we assume F-mode emission and iftp://mesola, obspm, fr/j rh_film/ 2http ://cdaw. gsfc. nasa. gov/CME_list/index, html
- 402 -
Radio Shocks from Reconnection Ouoqow J e t ? - New Observations
Fig. 2. Hard X-ray spectrometer (AO Ond~ejov) and radio data (AI Potsdam, AO Ond~ejov) of 29 March 2001. a) Three HXRS channels together with 3 GHz, 327 and 70 MHz plotted in arbitrary units, b) Dynamic radio spectrogram. Horizontal stripes result from strong interference. The black dashed box is enlarged in Figure 3a. Arrows point at other "HB" patches (see text). White dash-dotted line - the slowest drifting feature. a one-fold Newkirk density model. This means we find a reasonable density model transforming the burst drift rate into the independently determined CME speed. If this burst is a CME signature the delay between first faint drift bursts and the proper CME-lift-off is not unusual (Aurass et al. 1999). At 10:03- > 10:07 UT, we recognize F and H mode signatures of a "conventional" type II burst between 250 and 70 MHz. During the type II emission the noise storm starts to get depressed. Eventually, at 10:04 UT a slowly drifting and narrowband enhancement appears (190-260 k m s -1 with the Newkirk model; white dashdotted in Figure 2b). This feature resembles to the sawtooth pattern discovered by Klassen et al. (2001). From SOHO-EIT images we estimate an EIT wave speed of about 200 k m s -1 possibly corresponding to the slowest drifting radio feature. Furthermore, as shown in Figure 2b, there are patches of drift bursts between 250-800 MHz. At about 10:04:15 UT, one patch covers the starting point of the sawtooth-like slow drift burst. An F-H relation can not be recognized in these patches. But some of them seem to tend more towards lower, others more towards higher frequencies in the spectrum. An enlargement of one of these patches (Figure 3a) discloses that this can be HB emission. HB fine structure of type II bursts is due to nonthermal electrons energized at coronal shock waves. The HB patches are clearly not connected with the above mentioned low frequency ("conventional") type II burst. Because the patches itself do not show a well defined drift rate we argue for radio emission from one (or more) standing fast mode magnetosonic shocks such a sketched in Figure l a as FMSS. In Figure 3b we have plotted smoothed cuts through the HB patches at some representative frequencies. Superposed are three channels of the HXRS data. The figure demonstrates that the HB patches appear simultaneously with the HXR emission in the (most energetic) 100-147 keV channel of HXRS.
CONCLUSIONS We demonstrated the presence of patches of radio fine structures reminiscent of herringbones - a typical shock-excited radio fine structure - during the main maximum of energy release (shown by HXRS data) of a dynamic flare. This emission is observed in the decimeter range (here: 2 5 0 - > 800 MHz). As in Paper I we did not find a fundamental-harmonic mode pattern. We argue that these patches are excited by the perpendicular standing fast mode shocks sketched in Figure la. The simultaneous cusp formation and evolution (shown by SOHO-EIT data) supports our argument. Thus we extended the work started in - 403 -
H. Aurass et al.
Fig. 3. Decimetric HB emission, a) Enlargement of the dashed box in Figure 2b. Notice drift rates of both signs. b) Selected single frequency flux of smoothed HB patches (baseline shifted for clarity). Superposed (dashed) are 3 HXRS channels (compare with Figure 2a). Paper I by a well observed case of a probable decimetric standing shock signatures during the impulsive flare phase. The presented result confirms the assumption of Paper I that standing shock signatures in dynamic spectra are not rare but mostly not recognized as shock-driven emission due to insufficient sensitivity, time and frequency resolution of most sweep spectrometers, and due to previously missing supplementary observations in other spectral ranges. ACKNOWLEDGEMENTS HXRS is a joint endeavor by AICAS of the Czech Republic and SEC of NOAA, USA. The CME catalog is generated and maintained by the Center for Solar Physics and Space Weather 3. SOHO is a project of international cooperation between ESA and NASA. REFERENCES Aurass, H., A. Vourlidas, M.D. Andrews, B.J. Thompson, R.H. Howard, & G. Mann, Nonthermal Radio Signatures of Coronal Disturbances with and without CMEs, ApJ, 511, 451 (1999). Aurass, H., B. Vr~nak, & G. Mann, (Paper I), Shock-excited Radio Burst from Reconnection Outflow Jet? A 8JA, 384, 273 (2002). Chertok, I.M., S. Kahler, H. Aurass, & A.A. Gnezdilov, Sharp Decreases of Solar Metric Radio Storm Emission, Solar Phys., 202, 337 (2001). Delaboudini~re, J.P., et aL, EIT: EUV Imaging Telescope for SOHO, Solar Phys., 162, 291 (1995). Farnik, F., H. Garcia, & M. Karlicky, A New Solar Hard X-ray Spectrometer, Solar Phys., 201,357 (2001). Klassen, A., H. Aurass, & G. Mann, Sawtooth Oscillations in Solar Flare Radio Emission, A 8JA, 370, L41 (2001). Nelson, G.S. & D.B. Melrose, Type II Bursts, in Solar Radiophysics, eds. D.J. McLean, & N.R. Labrum, Cambridge Univ. Press, Cambridge, 333 (1985). Sato, T. & T. Hayashi, Externally Driven Magnetic Reconnection, Phys. Fluids, 22, 1189 (1979).
aThe Catholic University of America in cooperation with the Naval Research Laboratory and NASA. - 404 -