Radio emission in jets from young stellar objects (YSOs) in the form of nonthermal emission has been seen toward several YSOs. Thought to be synchrotron emission from strong shocks in the jet, it could provide valuable information about the magnetic field in the jet. Here we report on the detection of synchrotron emission in two emission knots in the jet of the low-mass YSO DG Tau A at 152 MHz using the Low-Frequency Array (LOFAR), the first time nonthermal emission has been observed in a YSO jet at such low frequencies. In one of the knots, a low-frequency turnover in its spectrum is clearly seen compared to higher frequencies. This is the first time such a turnover has been seen in nonthermal emission in a YSO jet. We consider several possible mechanisms for the turnover and fit models for each of these to the spectrum. Based on the physical parameters predicted by each model, the Razin effect appears to be the most likely explanation for the turnover. From the Razin effect fit, we can obtain an estimate for the magnetic field strength within the emission knot of $sim 20 mu mathrm{G}$. If the Razin effect is the correct mechanism, this is the first time the magnetic field strength along a YSO jet has been measured based on a low-frequency turnover in nonthermal emission.
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