INTERPLANETARY SHOCKS LACKING TYPE II RADIO BURSTS
- NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)
- Catholic University of America, Washington, DC 20064 (United States)
- Interferometrics, Herndon, VA 20170 (United States)
- Naval Research Laboratory, Washington, DC 20375 (United States)
We report on the radio-emission characteristics of 222 interplanetary (IP) shocks detected by spacecraft at Sun-Earth L1 during solar cycle 23 (1996 to 2006, inclusive). A surprisingly large fraction of the IP shocks ({approx}34%) was radio quiet (RQ; i.e., the shocks lacked type II radio bursts). We examined the properties of coronal mass ejections (CMEs) and soft X-ray flares associated with such RQ shocks and compared them with those of the radio-loud (RL) shocks. The CMEs associated with the RQ shocks were generally slow (average speed {approx}535 km s{sup -1}) and only {approx}40% of the CMEs were halos. The corresponding numbers for CMEs associated with RL shocks were 1237 km s{sup -1} and 72%, respectively. Thus, the CME kinetic energy seems to be the deciding factor in the radio-emission properties of shocks. The lower kinetic energy of CMEs associated with RQ shocks is also suggested by the lower peak soft X-ray flux of the associated flares (C3.4 versus M4.7 for RL shocks). CMEs associated with RQ CMEs were generally accelerating within the coronagraph field of view (average acceleration {approx}+6.8 m s{sup -2}), while those associated with RL shocks were decelerating (average acceleration {approx}-3.5 m s{sup -2}). This suggests that many of the RQ shocks formed at large distances from the Sun, typically beyond 10 Rs, consistent with the absence of metric and decameter-hectometric (DH) type II radio bursts. A small fraction of RL shocks had type II radio emission solely in the kilometric (km) wavelength domain. Interestingly, the kinematics of the CMEs associated with the km type II bursts is similar to those of RQ shocks, except that the former are slightly more energetic. Comparison of the shock Mach numbers at 1 AU shows that the RQ shocks are mostly subcritical, suggesting that they were not efficient in accelerating electrons. The Mach number values also indicate that most of these are quasi-perpendicular shocks. The radio-quietness is predominant in the rise phase and decreases through the maximum and declining phases of solar cycle 23. About 18% of the IP shocks do not have discernible ejecta behind them. These shocks are due to CMEs moving at large angles from the Sun-Earth line and hence are not blast waves. The solar sources of the shock-driving CMEs follow the sunspot butterfly diagram, consistent with the higher-energy requirement for driving shocks.
- OSTI ID:
- 21394456
- Journal Information:
- Astrophysical Journal, Vol. 710, Issue 2; Other Information: DOI: 10.1088/0004-637X/710/2/1111; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
COSMOLOGY AND ASTRONOMY
ACCELERATION
ELECTRONS
EMISSION
EXPLOSIONS
KINETIC ENERGY
MACH NUMBER
MASS
SHOCK WAVES
SOFT X RADIATION
SOLAR CYCLE
SOLAR WIND
SUN
SUNSPOTS
DIMENSIONLESS NUMBERS
ELECTROMAGNETIC RADIATION
ELEMENTARY PARTICLES
ENERGY
FERMIONS
IONIZING RADIATIONS
LEPTONS
MAIN SEQUENCE STARS
RADIATIONS
SOLAR ACTIVITY
STARS
STARSPOTS
STELLAR ACTIVITY
STELLAR WINDS
VELOCITY
X RADIATION