Postmidnight chorus: A substorm phenomenon


  • Bruce T. Tsurutani,

  • Edward J. Smith


The ELF (10–1500 Hz) electromagnetic emissions in the midnight sector of the outer magnetosphere have been studied using Ogo 5 search coil magnetometer data. Chorus was detected in conjunction with magnetospheric substorms throughout the region from L = 5 to L = 9 but only during postmidnight hours. No chorus was seen in the 3 hr preceding midnight even when substorms were in progress. The postmidnight chorus occurred only within ±15° of, and most frequently at, the geomagnetic equator. Time-averaged intensities varied from 10−8 to 10−6 γ²/Hz, which is more than an order below the maximum values reported previously for dayside chorus. The chorus occurred in narrow frequency bands having a bandwidth of 50–200 Hz. Chorus frequencies varied from less than one-fourth to as high as three-fourths of the equatorial electron gyrofrequency as determined by the on-board magnetometer. All frequencies in this range were detected except for a narrow band near one-half the electron gyrofrequency where the chorus appeared to be strongly attenuated. Chorus was often observed as two distinct bands above and below one-half the gyrofrequency. The two most common types of chorus were narrow band chorus without structure and falling tones. Rising tones and hooks were also observed. Chorus pulsations were observed often with quasi-periods of 5–15 s. A correlation was sought, but none was found, between the micropulsations in the ambient magnetic field and the chorus pulsations. Many features of postmidnight chorus can be explained by a cyclotron resonant interaction between the waves and the substorm electrons. The distribution of the chorus as a function of local time and L is strikingly similar to the distribution of enhanced, trapped, and precipitated substorm electrons with energies >40 keV. The postmidnight occurrence of chorus is attributable to the eastward curvature and gradient drift of the injected electrons. Cyclotron resonant interactions should be strongest near the equator, as was observed. The confinement to within 15° of the equator is attributed to Landau damping by low-energy (1–10 keV) auroral electrons. The attenuation band at one-half the electron gyrofrequency may result from Landau damping by electrons that have energy corresponding to cyclotron resonance but are traveling in the same direction as the waves. A close correspondence is expected between the occurrence of postmidnight and dayside chorus. The maximum in dayside chorus intensity at approximately 1000 LT, which is correlated with the dayside maximum of energetic electron precipitation, may represent further precipitation of the substorm electrons injected near midnight.