Electric field and plasma density measurements in the auroral electrojet

Abstract

Intense electrostatic waves in the auroral E region have been detected simultaneously on two payloads launched in a mother‐daughter configuration from Kiruna, Sweden. The data sets comprise electric field and density measurements from the ambient (dc) conditions to fluctuations as high as 50 kHz. The dc electric field measured by both payloads was 54 mV/m northwest, which corresponded to an electron drift velocity of 1080 m/s. This electric field drove two‐stream waves perpendicular to both B and E observed by both spacecraft throughout an altitude region which agrees quite well with the range predicted by linear two‐stream theory. The power in the waves depended on the electron density gradient, diminishing near 107 km where the gradient changed direction for a few kilometers. This observation is consistent with a gradient drift wave contribution to the instability process, since the auroral zone geometry does permit a component of the electric field perpendicular to B to be parallel to the vertical electron density gradient. Electric field spectra corroborate these results, as a strong component was detected at longer wavelengths (several hundred meters), the spectral regime associated with this instability. The spectra measured on both payloads also reveal an enhanced nearly coherent wave of a few meters wavelength at the topside of the layer (120 km), which also appears as electrostatic and parallel to the current. This spectral feature may be the consequence of a narrow range of wavelengths excited near the two‐stream instability threshold. Descending through the layer, the fluctuations fill out to a broadband, turbulent spectrum, extending to scale sizes down to the order of centimeters. Long wavelength waves form the strongest spectral component below 105 km. Power observed perpendicular to the direction of the current may indicate the presence of secondary plasma waves.

On the generation of large wave parallel electric fields responsible for electron heating in the high‐latitude E region

“…As we show below, the in situ rocket data [Pfaff et al, 1984;Rose, 1992] for meter-scale waves suggests a < 0.1. Because the RMS dE k derived from (1) rocket RMS dE ?…”

“…However, measurements of wave electric field at longer wavelengths are limited by in situ sensor attitude uncertainties in the case of rocket measurements and ionospheric refraction of radio signals in the case of radar measurements. Rocket electric field spectra appear to increase toward larger wavelengths [Pfaff et al, 1984;Rose, 1992], though the contribution to RMS dE ? is hard to quantify.…”

“…[26] There are indications [e.g., Pfaff et al, 1984;Rose, 1992] by rocket spectra that suggest that the spectrum increases toward larger wavelengths, implying that the RMS E ? is larger for 50-100 m waves.…”

“…The in situ perpendicular electric field turbulence measurements from various rockets are summarized in Table 1. The data are from the Porcupine II and ROSE rockets [Pfaff et al, 1984;Rose et al, 1992], an instrumented rocket launched from Manitoba, Canada [Kelley and Mozer, 1973], and an instrumented rocket launched from Sondrestrom, Greenland [Bahnsen et al, 1978]. The RMS drift values correspond to the approximate maximums of altitude profiles of the measured fluctuations.…”

On the spectrum of mid-latitude sporadic-&lt;i&gt;E&lt;/i&gt; irregularities

“…However, the approach developed here can be applied to the irregularities in the lower ionosphere at high and low latitudes. At least the present theoretical estimates of the spectral form and the rms level of plasma¯uctuations do not seem to contradict experimental results obtained during rocket measurements in the auroral and equatorial lower ionosphere (see, e.g., Rùyrvic and Smith, 1984;Pfa et al, 1984Pfa et al, , 1987aSchlegel, 1992;Gurevich et al, 1996;Ulwick et al, 1993;LuÈ bken et al, 1993). It should be noted that in some of the mentioned works (Pfa et al, 1984(Pfa et al, , 1987a plasma instabilities are proposed as basic mechanisms for the generation of ionospheric irregularities while in others a role of neutral turbulence in the formation of plasma irregularities is admitted (R yrvic and Smith, 1984;Schlegel, 1992;Gurevich et al, 1996;Ulwick et al, 1993;LuÈ bken et al, 1993).…”