Radio Emission from the Heliopause Triggered by an Interplanetary Shock Author(s): D. A. Gurnett, W. S. Kurth, S. C. Allendorf, R. L. Poynter Source: Science, New Series, Vol. 262, No. 5131 (Oct. 8, 1993), pp. 199-203 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/2882326 Accessed: 16/06/2010 15:06 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=aaas. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve and extend access to Science. http://www.jstor.org ARTICLE....- Radio Emission from the Heliopause Triggered by an InterplanetaryShock D. A. Gurnett,W. S. Kurth,S. C. Allendorf,R. L. Poynter A strong heliospheric radio emission event has been detected by Voyagers 1 and 2 in the locatedat R = 39.0 AU, e = -11.7?, and frequency range of 2 to 3 kilohertz. This event started in July 1992 and is believed to have X = 2830. The spectrogramscover a fre- been generated at or near the heliopause by an interplanetary shock that originated during quency range from 1 to 4 kHz and a time a period of intense solar activity in late May and early June 1991. This shock produced large spanof 1 year, fromday 120 of 1992 to day plasma disturbances and decreases in cosmic ray intensity at Earth, Pioneers 10 and 1 1, 120 of 1993. and Voyagers 1 and 2. The average propagation speed estimated from these effects is 600 Two primaryspectral components can to 800 kilometers per second. After correction for the expected decrease in the shock speed be seen in Fig. 1: a main emissionband at in the outer heliosphere, the distance to the heliopause is estimated to be between 1 16 about 2.0 kHz, and a seriesof narrowband and 177 astronomical units. emissionsdrifting upward in frequencyat a rate of about 3.0 kHz year-1, reaching a peakfrequency of about3.6 kHz. The main band has a sharplydefined low-frequency The heliopauseis the boundarybetween a was locatedat a heliocentricradial distance cutoff at 1.8 kHz. Both componentsmust hot (- 105 K) ionized gas flowingoutward of R = 50.8 AU and a solarecliptic latitude consist of electromagneticwaves because fromthe sun, called the solarwind (1), and and longitudeof 1B= 33.60 and A = 2450 they occur at frequencieswell above the the interstellarmedium, which is a relative- (as of 1 January1993), and Voyager2 was local plasma frequency. The plasma fre- ly cool (- 104 K), partially ionized gas between the stars.Because the sun is mov- ing with respect to the nearbyinterstellar Intensity(dB) medium,the heliopauseis expectedto form 0 10 a teardrop-shapedsurface, the nose of l I which is towardthe directionof arrivalof Voyager1 the interstellargas (2). The region inside the heliopause is called the heliosphere. 4.0 Estimatesof the distanceto the heliopause based on our limited knowledge of the interstellarmedium have variedfrom a few 3.0 tens to severalhundred astronomical units (1 AU = 1.49 x 108 km). For a recent review of the heliosphereand its interac- tion with the interstellarmedium, see Suess 2.0 (3). N Fourspacecraft, Pioneers 10 and 11 and I Voyagers1 and 2, areon escapetrajectories 1.0 fromthe sun to studythe outer heliosphere and to penetrateinto the interstellarmedi- um. None has yet reachedthe heliopauseor ) Voyager2 the solar wind terminalshock, which is a C-a) 4.0 standingshock that is expected to form in IL. the supersonicsolar wind flowwell insideof the heliopause. The radio emission de- 3.0 scribedin this reportand its interpretation provide a direct measurementof the dis- tance to the heliopause. 2.0 Description of the Event Frequency-timespectrograms of the radio 1.0 emissionevent (Fig. 1) were obtainedfrom the widebandplasma wave system (PWS) 120 180 240 300 360 0 60 120 on Voyager (4), which provides periodic 1992 1993 samplesof the electricfield waveform over a bandwidthof 50 Hz to 10 kHz. Voyager 1 Time (day and year) Fig. 1. Frequency-time spectrograms of the 1992-93 heliospheric radio emission event from (top) D. A. W. and in Gurnett, S. Kurth, S. C. Allendorf are the the Department of Physics and Astronomy, University Voyager 1 and (bottom) Voyager 2. The color indicates the electric field intensity, with red being of Iowa, Iowa City, IA 52242. R. L. Poynter is at the Jet most intense and blue being the least intense. Sampling times are indicated by marks at the top of Propulsion Laboratory, Pasadena, CA 91109. each panel. SCIENCE * VOL. 262 * 8 OCTOBER 1993 199 quency (f = 9000 VN Hz, whereN is the Day electroncfensity in electronsper cubic cen- = 8.0 _Complete rolls 3.11 kHz Roll-axis 311 (lookingtoward timeter)is the low-frequencycutoff of free- E Earth) , - _ 036 Source space electromagnetic waves and corre- __ 1 / _- directions 6.0- spondsapproximately to the high-frequency - - -, (3.1 kHz) limit of locallygenerated plasma waves. For _ _, f ~~~220 the time period of interest, the plasma p4.0- ' Sun's velocity relativeto frequency,as determinedby the Voyager2 - Voyager I nearby Fig. - interstellarmedium plasmainstrument, ranged from 0.4 to 1.3 Day220 kHz, with a mean of around0.7 kHz (5). 7 Aug 1992 Fig. 3. Source directions at 3.11 kHz deter- mined from roll maneuvers on days 220 and The exact startingtime of the radio emis- 'L 18 20 22 24 0 Time (hours) 311, 1992, and on day 36, 1993. The view is sion is difficultto determinefrom the wide- from the spacecraft looking along the roll axis A banddata because the time resolutionearly Fig. 2. plot of the electric field intensity in the (, = -33.7?, X = 63.70) toward the Earth. The in the event is only one spectrumper week 3.11 -kHz channel during a 10-turn roll maneu- horizontal (reference) direction has been taken (Fig. 1). A more accuratestarting time can ver on day 220, 1992 (11-point sliding average, to be the direction of the sun's motion with be obtained from the PWS 16-channel best fit marked by smooth curve). A clear respect to the nearby interstellar medium (p = roll-modulation signal can be seen. The phase 5.00, X = 2540), as given by Ajello et al. (24). spectrum-analyzerdata, which providesone of the roll modulation gives the direction to the measurementevery 16 s. These data show source. that the radioemission first appeared in the 1.78-kHz channel on day 188 (6 July) of consistent with a source location near ei- 1992. After the start of the event, the frequencyrange to detect the radio emis- ther the nose or the tail of the heliosphere. intensity graduallyincreased over a period sion. No roll-modulationeffects were ob- For the specific geometry involved, the of months, reachinga peakin earlyDecem- servedin the 1.78-kHzchannel. However, locationnear the nose or tail turnsout to be ber 1992. Thereafter,the intensity gradu- a clear roll-modulationsignal was observed nearly independentof the distance to the ally decreasedand was almostdown to the in the 3.11-kHz channel during all three source (<10 variationfor distancesgreater receiver-noiselevel as of July 1993. maneuvers(Fig. 2). The sinusoidalmodu- than 100 AU). The variationsin the ob- A strikingcharacteristic of the overall lation in the electricfield intensityat twice served source directionswere probablyre- event is the similarityof the spectrumsat the rotation period of the spacecraftis lated to the fact that the 3.1 1-kHz channel the two spacecraft,even though they are clearly evident. To determine the ampli- was respondingto the upward-driftingnar- separatedby 44.6 AU. This similaritysug- tude and phase of the modulation,we fit a rowbandfeatures (see Fig. 1), which were gests that the source is at a considerable sine function to the data using a least- clearly evolving during the course of the distance, greater than 44 AU. From the squares fitting procedure. All three roll event. maximum radiation intensity (1.8 x 10-17 maneuvershad similar modulation signa- W m-2 Hz-'), the radiated power is esti- tures.The peak-to-peakmodulation ampli- Relation to the Great Forbush mated to be at least 1013 W. This radio tudes range from about 10 to 20%. These Decrease of 1991 source is much strongerthan any known relativelylow modulationamplitudes indi- planetaryradio source and is probablythe cate that the sourceis eitherrelatively large Only one previousheliospheric radio emis- most powerfulradio emission in the solar (>600) or is locatedwell awayfrom the roll sion event has been observedwith intensi- system. axis (.40?), or a combination of these ties comparablewith those of the 1992-93 effects. The existence of a significantan- event. This event occurredin 1983-84 and Radio Direction Finding isotropyalso indicatesthat the radiation(at was firstdescribed by Kurthet al.